Selling, general and administrative expenses were $492.0, or 12.2% of net sales, and $1,340.4, or 12.3% of net sales, for the third quarter and first nine months of 2024, respectively, compared to $381.6, or 11.9% of net sales, and $1,095.7, or 11.9% of net sales, for the third quarter and first nine months of 2023, respectively. The increases in selling, general and administrative expenses as a percentage of net sales in the third quarter and first nine months of 2024 were primarily driven by the effect of acquisitions, which currently have higher selling, general and administrative expenses as a percentage of net sales compared to the Company average. Administrative expenses represented approximately 5.0% and 4.9% of net sales for the third quarter and first nine months of 2024, respectively, and represented approximately 4.9% and 4.8% of net sales for the third quarter and first nine months of 2023, respectively. Research and development expenses represented approximately 3.1% and 3.0% of net sales for the third quarter and first nine months of 2024, respectively, and represented approximately 2.7% of net sales for both the third quarter and first nine months of 2023. Selling and marketing expenses represented approximately 4.1% and 4.3% of net sales for the third quarter and first nine months of 2024, respectively, and represented approximately 4.3% of net sales for both the third quarter and first nine months of 2023.
Operating income was $819.5, or 20.3% of net sales, and $2,203.2, or 20.2% of net sales, for the third quarter and first nine months of 2024, respectively, compared to $657.9, or 20.6% of net sales, and $1,869.6, or 20.3% of net sales, for the third quarter and first nine months of 2023, respectively. Operating income for the third quarter and the first nine months of 2024 includes $63.6 and $133.6, respectively, of acquisition-related expenses, comprised primarily of (i) the amortization related to the value associated with acquired backlog resulting from the Carlisle Interconnect Technologies (“CIT”) acquisition and external transaction costs associated with acquisitions (such acquisition-related expenses aggregating $45.4 and $115.4, respectively, are presented separately in the Condensed Consolidated Statements of Income) and (ii) the amortization of acquisition-related inventory step-up costs in the third quarter of 2024 of $18.2 associated with the CIT acquisition (such costs are recorded in Cost of sales in the Condensed Consolidated Statements of Income). Operating income for the third quarter of 2023 included $9.0 of acquisition-related expenses, comprised of external transaction costs related to acquisitions. Operating income for the first nine months of 2023 included $18.4 of acquisition-related expenses, comprised of external transaction costs incurred in the second and third quarters of 2023, as well as the amortization related to the value associated with acquired backlog resulting from an acquisition that closed in the first quarter of 2023. Acquisition-related expenses during the three and nine months ended September 30, 2023 are presented separately in the Condensed Consolidated Statements of Income. For the three and nine months ended September 30, 2024, the acquisition-related expenses had the effect of decreasing net income by $49.8, or $0.04 per share, and $109.7, or $0.09 per share, respectively, while for the three and nine months ended September 30, 2023, the acquisition-related expenses had the effect of decreasing net income by $8.4, or $0.01 per share, and $16.2, or $0.01 per share, respectively, as shown in the tables below. Excluding the effect of these acquisition-related expenses, Adjusted Operating Income and Adjusted Operating Margin, each as defined in the “Non-GAAP Financial Measures” section below, were $883.1, or 21.9% of net sales, and $666.9, or 20.8% of net sales, for the three months ended September 30, 2024 and 2023, respectively. Excluding the effect of these acquisition-related expenses, Adjusted Operating Income and Adjusted Operating Margin were $2,336.8, or 21.4% of net sales, and $1,888.0, or 20.5% of net sales, for the nine months ended September 30, 2024 and 2023, respectively. The increases in Adjusted Operating Income and Adjusted Operating Margin for both the third quarter and first nine months of 2024 relative to the comparable periods in 2023 were primarily driven by strong operating performance on the higher sales volumes, partially offset by the negative impact on operating margin related to acquisitions completed within the prior twelve months that are currently operating below the average operating margin of the Company.
Operating income for the Harsh Environment Solutions segment for the third quarter and first nine months of 2024 was $283.7, or 23.8% of net sales, and $787.8, or 25.0% of net sales, respectively, compared to $239.1, or 26.9% of net sales, and $705.7, or 26.8% of net sales, for the third quarter and first nine months of 2023, respectively. The decreases in operating margin for the Harsh Environment Solutions segment for both the third quarter and first nine months of 2024 relative to the comparable periods in 2023 were primarily driven by the negative impact on operating margin related to acquisitions completed within the prior twelve months, particularly the CIT acquisition, that are currently operating below the average operating margin of the Company.