on a tax-equivalent (TE) basis, expressed as a percentage of average earning assets, was 3.28% for the third quarter of 2020, reflecting a 20 basis point decrease from the second quarter of 2020, and an 86 basis point decrease from the third quarter of 2019. The average tax-equivalent yield on earning assets decreased to 3.65% for the third quarter of 2020, compared to 3.93% for the second quarter of 2020, and 4.90% for the third quarter of 2019. The decreases in net interest margin and the yield on average earning assets for the third quarter of 2020, compared to the third quarter of 2019, was primarily attributable to a decline in security volumes and interest rate reductions which impacted loans and securities in the third quarter of 2020. Average PPP loans for the third quarter of 2020 totaled $136.3 million, which resulted in a reduction to our net interest margin (TE) of six basis points for the quarter as these loans were issued at a 1.00% interest rate, plus origination fees, net of costs. The cost of funds on interest bearing liabilities was 0.58% for the third quarter of 2020, 0.72% for the second quarter of 2020, and 1.11% for the third quarter of 2019. The decrease in our cost of funds in the third quarter of 2020 compared to the second quarter of 2020 was primarily driven by a full quarter of impact of interest rate reductions on our deposits. The decrease in our cost of funds in the third quarter of 2020 compared to the third quarter of 2019 was primarily driven by a decline in the rates paid on deposits, a decline in volume and rates paid on short-term borrowings, and the redemption of our junior subordinated debentures in the first quarter of 2020 which resulted in the average yield on our junior subordinated debentures of 4.40% for the third quarter of 2020, compared to 6.41% in the prior year like quarter.
Nine months ended September 30, 2020 and 2019
Our net interest and dividend income decreased by $5.7 million to $67.9 million for the nine months ended September 30, 2020, compared to $73.6 million for the nine months ended September 30, 2019. This decline was attributable to a $9.7 million, or 11.1%, decrease in interest and dividend income for the nine months ended September 30, 2020, compared to the nine months ended September 30, 2019, partially offset by a $4.0 million decrease in interest expense.
Average earning assets for the nine months ended September 30, 2020 were $2.63 billion, reflecting an increase of $197.0 million, or 8.1%, compared to the nine months ended September 30, 2019. The yield on average earning assets for the nine months ended September 30, 2020 was 4.02%, compared to 4.91% for the nine months ended September 30, 2019. Total average loans, including loans held-for-sale, totaled $2.02 billion for the nine months ended September 30, 2020, which reflected an increase of $119.5 million compared to the nine months ended September 30, 2019. The growth in average loan balances was more than offset by market interest rate reductions, which resulted in a $6.1 million decrease in loan interest income for the nine months ended September 30, 2020, compared to the like 2019 period. For the nine months ended September 30, 2020, yields on average securities decreased by 71 basis points and yields on average loans decreased by 72 basis points, each as compared to the nine months ended September 30, 2019, due primarily to the falling interest rate environment. Average interest earning deposits with financial institutions increased $128.9 million in the nine months ended September 30, 2020, compared to the prior year like period as consumer spending declined year over year as a result of the COVID-19 pandemic, which increased our liquidity.
Average interest bearing liabilities decreased $23.2 million, or 1.4%, in the nine months ended September 30, 2020, compared to the nine months ended September 30, 2019. The decrease was due to reductions in average other short-term borrowings of $75.7 million, which primarily consist of FHLBC advances, and a reduction in average junior subordinated debentures of $24.8 million due to the redemption of the trust preferred securities and the related $32.6 million of subordinated debentures issued by Old Second Capital Trust I in March 2020. The rate on our junior subordinated debentures was 7.85% for the nine months ended September 30, 2020, and 6.47% for the nine months ended September 30, 2019, as $635,000 of deferred issuance costs was expensed upon the redemption in 2020. Average interest bearing deposits increased by $61.9 million in the nine months ended September 30, 2020, compared to the nine months ended September 30, 2019, but the reduction in market interest rates resulted in a 16 basis point reduction in the cost of interest bearing deposits for the nine months ended September 30, 2020, compared to the like 2019 period. Average noninterest bearing deposits increased $167.2 million in the nine months ended September 30, 2020, compared to the nine months ended September 30, 2019, as customers spent less as a result of the COVID-19 pandemic.
Our net interest margin for the nine months ended September 30, 2020 was 3.44% compared to 4.04% for the nine months ended September 30, 2019, reflecting a 60 basis point decrease. Our net interest margin (TE) for the nine months ended September 30, 2020 was 3.50% compared to 4.13% for the nine months ended September 30, 2019, reflecting a 63 basis point decrease. The decrease in net interest margin for the nine months ended September 30, 2020, compared to the nine months ended September 30, 2019, was due primarily to reductions in the market interest rates as well as the issuance of $136.7 million of PPP loans at 1.00%, and the growth of our liquidity with an increase in cash and due from banks, which have not yet been reinvested into higher yielding assets. We are currently assessing potential investment opportunities for this excess liquidity. Core net interest margin (TE), a non-GAAP financial measure that excludes PPP loans, was 3.54% for the nine months ended September 30, 2020, reflecting a four basis point reduction from net interest margin (TE) for the same period. These reductions to the net interest margin were partially offset by reductions in rates paid on deposits, as well as reductions in volumes and rates paid on borrowings, and growth in noninterest bearing deposits, which drove down our overall cost of funds.