Combined, net occupancy expense of premises and furniture and equipment expense was $7.0 million for the three months ended September 30, 2021, a 6.4% increase from the comparable period in 2020, and was $19.9 million for the nine months ended September 30, 2021, a 4.0% decrease from the comparable period in 2020. GSB added 7 branches in 2021. The Company closed 12 banking centers in October 2020, and has evaluated and expects to close and consolidate 17 Busey Bank banking centers, two of which were formerly GSB banking centers, in the fourth quarter of 2021.
Professional fees were $1.4 million for the three months ended September 30, 2021, a 19.8% decrease from the comparable period in 2020, and were $5.6 million for the nine months ended September 30, 2021, a 2.0% increase from the comparable period in 2020. Non-operating expenses contributed $0.1 million and $1.4 million in professional fees for the three and nine months ended September 30, 2021, compared to $0.2 million for the three and nine months ended September 30, 2020.
Amortization of intangible assets was $3.1 million for the three months ended September 30, 2021, a 26.3% increase from the comparable period in 2020, and was $8.2 million for the nine months ended September 30, 2021, an 8.3% increase from the comparable period for 2020. The increase primarily related to intangibles acquired in the CAC acquisition.
Interchange expense was $1.4 million for the three months ended September 30, 2021, a 17.3% increase from the comparable period in 2020, and was $4.4 million for the nine months ended September 30, 2021, a 21.4% increase from the comparable period in 2020. Fluctuations in interchange expense were primarily the result of increased payment and volume activity at FirsTech.
Other expense was $10.8 million for the three months ended September 30, 2021, a 39.1% increase from the comparable period in 2020, and was $28.4 million for the nine months ended September 30, 2021, a 13.9% increase from the comparable period in 2020. Increases were across multiple expense categories including New Market Tax Credit amortization, regulatory expenses, marketing, business development, recruiting and onboarding, director compensation, and card service fees, partially offset by lower MSR valuation impairment and releases in the provision for unfunded commitments.
The efficiency ratio(1), which is a measure commonly used by management and the banking industry, measures the amount of expense incurred to generate a dollar of revenue. The efficiency ratios were 67.3% and 61.4% for the three and nine months ended September 30, 2021, respectively, compared to 52.4% and 54.3% for the three and nine months ended September 30, 2020, respectively.
The adjusted efficiency ratios(1) were 59.0% and 57.5% for the three and nine months ended September 30, 2021, respectively, compared to 50.0% and 53.2% for three and nine months ended September 30, 2020, respectively. The Company remains focused on expense discipline.
Income Taxes
The effective income tax rates of 19.9% and 20.5% for the three and nine months ended September 30, 2021, respectively, were lower than the combined federal and state statutory rate of approximately 28% due to tax exempt interest income, such as municipal bond interest and bank owned life insurance income, and investments in various federal and state tax credits, including an Illinois new market tax credit. The Company continues to monitor evolving federal and state tax legislation and its potential impact on operations on an ongoing basis. As of September 30, 2021, the Company was not under examination by any tax authority.
(1) A Non-GAAP financial measure. See “Non-GAAP Financial Information” for reconciliation.