We use Gaussian-4 theory to investigate the reaction mechanism for the conversion of a 2-methylstyrene-based Criegee intermediate into o-toluic acid and 2-methylphenylformate. o-Toluic acid can be formed via an α-hydroxyalkyl-hydroperoxide intermediate with an activation energy of ΔG‡298 = 82.9 kJ mol−1 for the rate-determining-step (RDS). The RDS for the formation of 2-methylphenylformate has an activation energy of ΔG‡298 = 61.9 kJ mol−1. Formation of the o-toluic acid product is more exergonic by 67.4 kJ mol−1. Consistent with recent experimental results, our high-level calculations show that o-toluic acid is the thermodynamic product and 2-methylphenylformate is the kinetic product.