administrative expenses decreased from 32.2 percent of revenues in the second quarter of 2020 to 15.6 percent of revenues in the second quarter of 2021 due to the fixed nature of many of these expenses during the short term.
Depreciation and amortization. Depreciation and amortization decreased 8.6 percent to $17.9 million for the three months ended June 30, 2021, compared to $19.6 million for the three months ended June 30, 2020. Depreciation and amortization decreased due lower capital expenditures in recent years, coupled with assets becoming fully depreciated for book purposes during the previous quarters.
Impairment and other charges. There were no impairment and other charges for the three months ended June 30, 2021 and $1.6 million for the three months ended June 30, 2020. See Note 4 of the notes to the consolidated financial statements for a detail of these charges.
Gain on disposition of assets, net. Gain on disposition of assets, net was $3.1 million for the three months ended June 30, 2021 compared to a gain on disposition of assets, net of $3.2 million for the three months ended June 30, 2020. The gain on disposition of assets, net is generally comprised of gains and losses related to various property and equipment dispositions or sales to customers of lost or damaged rental equipment.
Other income (expense), net. Other income, net was $616 thousand for the three months ended June 30, 2021 compared to other expense, net of $1.5 million for the same period in the prior year.
Interest expense. Interest expense was $103 thousand for the three months ended June 30, 2021 compared to $71 thousand for the three months ended June 30, 2020. Interest expense includes facility fees on the unused portion of the credit facility and the amortization of loan costs.
Income tax provision (benefit). Income tax provision was $33 thousand during the three months ended June 30, 2021 compared to a $13.9 million income tax benefit for the same period in 2020. The effective tax rate was 4.8 percent for the three months ended June 30, 2021 compared to a 35.7 percent effective benefit rate for the three months ended June 30, 2020. The effective rate reflects a provision due to a net detrimental impact related to the employee retention credit.
SIX MONTHS ENDED JUNE 30, 2021 COMPARED TO SIX MONTHS ENDED JUNE 30, 2020
Revenues. Revenues of $371.4 million for the six months ended June 30, 2021 increased 11.5 percent compared to the six months ended June 30, 2020. Domestic revenues of $354.5 million increased 14.2 percent for the six months ended June 30, 2021 compared to the same period in the prior year. The increase in revenues was due to higher activity levels compared to the prior year which was negatively impacted during the second quarter of 2020 by COVID-19 shutdowns. International revenues of $16.9 million decreased 25.1 percent for the six months ended June 30, 2021 compared to the same period in the prior year.
During the first six months of 2021, the average price of natural gas was 81.0 percent higher and the average price of oil was 67.3 percent higher, both as compared to the same period in the prior year. The average domestic rig count during the first six months of 2021 was 27.8 percent lower than the same period in 2020.
The Technical Services segment revenues for the first six months of 2021 increased by 13.1 percent compared to the same period of the prior year due to higher activity levels. Technical Services reported an operating loss of $4.3 million during the first six months of 2021 compared to an operating loss of $46.3 million for the first six months of 2020. The Support Services segment revenues for the first six months of 2021 decreased by 9.0 percent compared to the same period in the prior year. This decrease was due principally to lower activity levels for rental tools. Support Services reported an operating loss of $5.3 million for the first six months of 2021 compared to operating loss of $0.3 million for the first six months of 2020 due to lower revenues from reduced activity levels.
Cost of revenues. Cost of revenues increased 11.5 percent to $292.0 million for the six months ended June 30, 2021 compared to $262.0 million for the six months ended June 30, 2020. Cost of revenues increased primarily due to increases in expenses consistent with higher activity levels, partially offset by efficiencies resulting from RPC’s cost reduction initiatives. Cost of revenues as a percentage of revenues was essentially unchanged in the first six months of 2021 as compared to the prior year.
Selling, general and administrative expenses. Selling, general and administrative expenses were $60.0 million for the six months ended June 30, 2021 and $65.3 million for the six months ended June 30, 2020. These expenses decreased due to lower employment costs, primarily the result of cost reduction initiatives during previous quarters. Selling, general and administrative expenses decreased