second quarter of 2023 primarily due to improved pricing for our services as well as reduced maintenance expense due to a decrease in the average age of our equipment.
Selling, general and administrative expenses. Selling, general and administrative expenses increased to $43.6 million for the three months ended June 30, 2023 compared to $35.9 million for the three months ended June 30, 2022, primarily due to costs related to the settlement of a vendor dispute and the acquisition of Spinnaker Oilwell Services. Selling, general and administrative expenses increased from 9.6 percent of revenues in the second quarter of 2022 to 10.5 percent of revenues in the second quarter of 2023.
Pension settlement charges. Pension settlement charges were $911 thousand for the three months ended June 30, 2023. There were no pension settlement charges for the three months ended June 30, 2022. See note 10 of the notes to the consolidated financial statements for more information.
Depreciation and amortization. Depreciation and amortization increased 30.4 percent to $26.2 million for the three months ended June 30, 2023, compared to $20.1 million for the three months ended June 30, 2022. Depreciation and amortization increased due to capital expenditures in the past year.
Gain on disposition of assets, net. Gain on disposition of assets, net was $3.0 million for the three months ended June 30, 2023 compared to a gain on disposition of assets, net of $1.8 million for the three months ended June 30, 2022. 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 $631 thousand for the three months ended June 30, 2023 compared to other income, net of $79 thousand for the same period in the prior year.
Interest expense and interest income. Interest expense was $73 thousand for the three months ended June 30, 2023 compared to $222 thousand for the three months ended June 30, 2022. Interest expense includes facility fees on the unused portion of the credit facility and the amortization of loan costs. Interest income increased to $2.7 million compared to $128 thousand in the prior year due to a higher average cash balance coupled with higher investment yields.
Income tax provision. Income tax provision was $20.6 million during the three months ended June 30, 2023 compared to $13.5 million tax provision for the same period in 2022. The effective tax rate was 24.1 percent for the three months ended June 30, 2023 compared to a 22.3 percent for the three months ended June 30, 2022. The increase in the 2023 effective tax rate is primarily due to unfavorable permanent items.
SIX MONTHS ENDED JUNE 30, 2023 COMPARED TO SIX MONTHS ENDED JUNE 30, 2022
Revenues. Revenues of $892.5 million for the six months ended June 30, 2023 increased 35.2 percent compared to the six months ended June 30, 2022. Domestic revenues of $878.8 million increased 36.4 percent for the six months ended June 30, 2023 compared to the same period in the prior year. The increase in revenues was primarily due to improved pricing, higher customer activity levels and a larger active fleet of pressure pumping equipment. International revenues of $13.7 million decreased 14.1 percent for the six months ended June 30, 2023 compared to the same period in the prior year.
During the first six months of 2023, the average price of oil was 26.7 percent lower, and the average price of natural gas was 60.4 percent lower, both as compared to the same period in the prior year. The average domestic rig count for the six months ended June 30, 2023 was 9.1 percent higher than the same period in 2022.
The Technical Services segment revenues for the first six months of 2023 increased by 35.3 percent compared to the same period of the prior year due to higher customer activity levels, improved pricing and a larger fleet of pressure pumping equipment in service. Technical Services reported operating income of $180.6 million during the first six months of 2023 compared to operating income of $81.6 million in the same period of 2022. The Support Services segment revenues for the first six months of 2023 increased by 34.1 percent compared to the same period in the prior year, primarily due to higher activity levels and improved pricing within rental tools. Support Services reported operating income of $14.6 million for the first six months of 2023 compared to an operating income of $6.1 million for the same period of 2022.
Cost of revenues. Cost of revenues increased 21.6 percent to $571.0 million for the six months ended June 30, 2023 compared to $469.8 million for the six months ended June 30, 2022. Cost of revenues increased primarily due to increases in expenses consistent