Combined, net occupancy expense of premises and furniture and equipment expense decreased by 9.6% to $6.3 million for the three months ended June 30, 2021, compared to $7.0 million for the same period in 2020. Combined, net occupancy expense of premises and furniture and equipment expense decreased by 8.8% to $12.9 million for the six months ended June 30, 2021, compared to $14.1 million for the same period in 2020. These decreases in 2021 were primarily related to savings achieved through the closure of 12 banking centers in October of 2020. On July 27, 2021, the Company announced its Personal Banking Transformation Plan to close and consolidate 15 Busey Bank banking centers as well as two GSB banking centers to be consolidated as part of the acquisition integration plan, with the banking center closures expected to occur in the fourth quarter of 2021.
Professional fees increased by 16.4% to $2.3 million for the three months ended June 30, 2021, compared to $2.0 million for the same period of 2020. Professional fees increased by 11.7% to $4.3 million for the six months ended June 30, 2021, compared to $3.8 million for the same period of 2020. Professional fee variances were largely influenced by acquisition expenses. Results for the three and six months ended June 30, 2021, include $0.9 million and $1.3 million, respectively, of non-operating professional fee expenses related to the acquisition.
Amortization of intangible assets increased by 5.2% to $2.7 million for the three months ended June 30, 2021, compared to $2.5 million for the same period in 2020. Amortization of intangible assets was $5.1 million for the six months ended June 30, 2021 and 2020. Results for the three and six months ended June 30, 2021, include $0.3 million of amortization expense related to GSB.
Interchange expense increased by 20.4% to $1.4 million for the three months ended June 30, 2021, compared to $1.2 million for the same period in 2020. Interchange expense increased by 23.6% to $2.9 million for the six months ended June 30, 2021, compared to $2.4 million for the same period in 2020. Fluctuations in interchange expense were primarily the result of increased payment and volume activity at FirsTech.
Other expense increased by 31.2% to $10.2 million for the three months ended June 30, 2021, compared to $7.8 million for the same period in 2020. Other expense increased by 2.6% to $17.6 million for the six months ended June 30, 2021, compared to $17.2 million for the same period in 2020. Increases were across multiple expense categories, including New Market Tax Credit amortization and business development expenses, partially offset by lower MSR valuation impairment and provision for unfunded commitments. Also contributing to the increase, deferral of PPP loan origination costs lowered other expenses by $0.1 million and $0.6 million for the three and six months ended June 30, 2021, respectively, compared to $1.1 million during the three and six months ended June 30, 2020. Results for the three and six months ended June 30, 2021, include one month of other expenses for GSB totaling $0.4 million.
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 ratio was 61.7% for the three months ended June 30, 2021, compared to 51.0% for the three months ended June 30, 2020. The efficiency ratio was 58.2% for the six months ended June 30, 2021, compared to 55.3% for the same period in 2020.
The adjusted efficiency ratio(1) was 58.9% for the three months ended June 30, 2021, compared to 50.5% for the three months ended June 30, 2020. The adjusted efficiency ratio was 56.6% for the six months ended June 30, 2021, compared to 55.0% for the same period in 2020. The Company remains focused on expense discipline.
Income Taxes
The effective income tax rates of 18.7% and 20.7% for the three and six months ended June 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 June 30, 2021, the Company was not under examination by any tax authority; however, Banc Ed, which the Company acquired on January 31, 2019, is under examination by the Illinois Department of Revenue for its 2009 to 2016 income tax filings.
(1) A Non-GAAP financial measure. See “Non-GAAP Financial Information” for reconciliation.