pricing, lower activity levels, and an unfavorable materials mix within pressure pumping, which is RPC’s largest service line. International revenues of $35.5 million decreased 17.3 percent for the six months ended June 30, 2019 compared to the same period in the prior year. Our international revenues are impacted by the timing of project initiations and their ultimate duration and can be difficult to predict.
During the six months ended June 30, 2019, the average price of natural gas was 8.7 percent lower and the average price of oil was 12.6 percent lower both as compared to the same period in the prior year. The average domestic rig count during the six months ended June 30, 2019 was 1.0 percent higher than the same period in 2018.
The Technical Services segment revenues for the six months ended June 30, 2019 decreased 25.0 percent compared to the same period in the prior year due to lower pricing and activity levels within most of the service lines which comprise this segment. The Support Services segment revenues for the six months ended June 30, 2019 increased by 16.2 percent compared to the same period in the prior year. This increase was due principally to improved activity levels across all services lines within this segment. Technical Services reported significantly lower operating profit of $2.4 million during the six months ended June 30, 2019 compared to $140.6 million in the same period of the prior year due to lower pricing and activity levels. Support Services reported higher operating profit of $7.2 million for the six months ended June 30, 2019 compared to $0.3 million for the six months ended June 30, 2018.
Cost of revenues. Cost of revenues decreased 14.8 percent to $517.5 million for the six months ended June 30, 2019 compared to $607.7 million for the six months ended June 30, 2018. Cost of revenues decreased, consistent with lower activity levels, due to lower materials and supplies expenses within RPC’s pressure pumping service line, as well as lower fuel costs and maintenance and repairs expense. As a percentage of revenues, cost of revenues increased in the six months ended June 30, 2019 compared to the same period in the prior year, due to lower revenues, increasingly competitive pricing for our services, and labor cost inefficiencies.
Selling, general and administrative expenses. Selling, general and administrative expenses were $88.7 million for the six months ended June 30, 2019 and $86.3 million for the six months ended June 30, 2018. As a percentage of revenues, these costs increased to 12.8 percent in the six months ended June 30, 2019 compared to 9.5 percent in the six months ended June 30, 2018 due to the leverage of lower revenues over primarily fixed expenses.
Depreciation and amortization. Depreciation and amortization increased 10.1 percent to $85.4 million for the six months ended June 30, 2019, compared to $77.6 million for the quarter ended June 30, 2018 due to capital expenditures made during the previous four quarters.
Gain on disposition of assets, net. Gain on disposition of assets, net increased to $4.6 million for the six months ended June 30, 2019 compared to $3.2 million for the six months ended June 30, 2018. 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, net. Other income, net was $0.4 million for the six months ended June 30, 2019 compared to $9.5 million for the same period in the prior year. Other income recorded in the six months ended June 30, 2018 included property insurance proceeds of approximately $9.6 million.
Interest expense. Interest expense was $253 thousand for the six months ended June 30, 2019 compared to $218 thousand for the six months ended June 30, 2018. Interest expense consists of facility fees on the unused portion of the credit facility and the amortization of loan costs.
Income tax provision. Income tax provision was $2.3 million during the six months ended June 30, 2019 compared to $33.9 million for the same period in 2018. The effective tax rate was 30.0 percent for the six months ended June 30, 2019 compared to 23.2 percent for the six months ended June 30, 2018. The 2019 effective rate includes detrimental discrete adjustments of $2.2 million resulting from restricted stock vesting and dividends as compared to the 2018 effective rate.