In situ attenuated total reflection infrared spectroscopy (ATR-IR) in the liquid phase and in situ transmission IR spectroscopy in the gas phase were used to validate a previously proposed mechanism for limonene epoxidation with aqueous hydrogen peroxide on PW-Amberlite. Catalyst activation by the oxidant was evidenced by ATR-IR through the presence of multiple bands in the 1220-910 cm(-1) range, which are attributed to metal-oxygen interactions on the catalytic surface. The presence of reaction intermediates is linked to bands in the 840-820 cm(-1) region. Gas phase experiments indicate that acetonitrile is easily adsorbed and desorbed from the catalyst, while limonene is adsorbed on the solvent or oxidant saturated PW-Amberlite and not on the fresh catalyst, as proposed in the mechanistic pathway. (C) 2009 Elsevier B.V. All rights reserved.