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STOCK-BASED COMPENSATION (Tables) | 9 Months Ended
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Sep. 30, 2025
Share-Based Payment Arrangement [Abstract] |
Stock-based compensation expense | Stock-based compensation expense was as follows:
| Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2025 |  | 2024 |  | 2025 |  | 2024
Stock Compensation Expense | ($ in millions)
Stock-based compensation | $ | 6.3 |  |  | $ | 6.3 |  |  | $ | 20.8 |  |  | $ | 17.3 |
Mark-to-market adjustments | 2.3 |  |  | 0.3 |  |  | (3.4) |  |  | (2.9) |
Total expense | $ | 8.6 |  |  | $ | 6.6 |  |  | $ | 17.4 |  |  | $ | 14.4 |
Schedule of fair value of performance share awards granted valuation assumptions | The fair value of each performance share award based on TSR was estimated on the date of grant, using a Monte Carlo simulation model with the following weighted average assumptions:
Grant Date Assumptions - Performance Shares | 2025 |  | 2024
Risk-free interest rate | 4.27 | % |  | 4.53 | %
Expected volatility of Olin common stock | 37 | % |  | 41 | %
Expected average volatility of peer companies | 35 | % |  | 37 | %
Average correlation coefficient of peer companies | 0.45 |  | 0.40
Expected life (years) | 3.0 |  | 3.0
Grant date fair value (TSR-based award) | $ | 27.46 |  | $ | 72.80
Grant date fair value (net income-based award) | $ | 27.62 |  | $ | 54.07
Performance share awards granted | 573,479 |  | 180,714

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- Definition Tabular disclosure of share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -SubTopic 10 -Topic 718 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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