Support Services reported operating income of $21.4 million for the first nine months of 2023 compared to operating income of $11.4 million for the same period of 2022.
Cost of revenues. Cost of revenues increased 3.9 percent to $810.1 million for the nine months ended September 30, 2023 compared to $779.5 million for the nine months ended September 30, 2022. Cost of revenues increased primarily due to increases in expenses consistent with higher activity levels, such as materials and supplies expenses, maintenance and repairs expenses, employment costs and fuel costs. Cost of revenues as a percentage of revenues, decreased from 69.6 percent for the nine months ended September 30, 2022 to 66.2 percent for the nine months ended September 30, 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. In accordance with SAB Topic 11.B, cost of revenues presented on the Consolidated Statements of Operations excludes depreciation and amortization totaling $71.2 million for the nine months ended September 30, 2023, compared to $53.9 million for the nine months ended September 30, 2022.
Selling, general and administrative expenses. Selling, general and administrative expenses increased to $127.8 million for the nine months ended September 30, 2023 compared to $110.4 million for the nine months ended September 30, 2022, primarily due to costs related to the settlement of a vendor dispute coupled with expenses incurred from recently acquired Spinnaker, as well as increases in variable costs consistent with higher activity levels. Selling, general and administrative expenses, as a percentage of revenues, increased from 9.9 percent in the first nine months of 2022 to 10.5 percent in the same period of 2023 primarily due to the settlement of a vendor dispute and the acquisition of Spinnaker during the first nine months of 2023.
Pension settlement charges. Pension settlement charges were $18.3 million for the nine months ended September 30, 2023. There was no pension settlement charge for the nine months ended September 30, 2022. See note 14 of the notes to the Consolidated Financial Statements for more information.
Depreciation and amortization. Depreciation and amortization increased 30.1 percent to $78.7 million for the nine months ended September 30, 2023, compared to $60.5 million for the nine months ended September 30, 2022. Depreciation and amortization increased due to capital expenditures in the past year coupled with additional depreciation from the acquisition of Spinnaker.
Gain on disposition of assets, net. Gain on disposition of assets, net was $7.7 million for the nine months ended September 30, 2023 compared to a gain on disposition of assets, net of $6.3 million for the nine months ended September 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 $2.2 million for the nine months ended September 30, 2023 compared to other income, net of $516 thousand for the same period in the prior year.
Interest expense and interest income. Interest expense was $246 thousand for the nine months ended September 30, 2023 compared to $543 thousand for the nine months ended September 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 $6.0 million compared to $472 thousand in the prior year due to a higher average cash balance coupled with higher investment yields.
Income tax provision. Income tax provision was $48.8 million during the nine months ended September 30, 2023 compared to $44.7 million tax provision for the same period in 2022. The effective tax rate was 24.0 percent for the nine months ended September 30, 2023 compared to a 25.4 percent effective tax rate for the nine months ended September 30, 2022. The decrease in the 2023 effective tax rate is primarily due to smaller unfavorable discrete adjustments.
Liquidity and Capital Resources
Cash Flows
The Company’s cash and cash equivalents increased $45.5 million to $171.9 million as of September 30, 2023 compared to cash and cash equivalents of $126.4 million as of December 31, 2022. This increase is primarily due to favorable changes in working capital coupled with an increase in net income during 2023 compared to the prior year, partially offset by the cash acquisition of Spinnaker during 2023.
The following table sets forth the historical cash flows for the nine months ended September 30, 2023 and 2022: