The conversion of cyclic ketones to lactones was catalyzed by different acid catalysts using aqueous hydrogen peroxide (30 wt%) as a clean oxidant. H3PW12O40 seems to be a rather efficient and highly selective catalyst for the Baeyer-Villiger reaction of 2-heptylcyclopentanone and 2-adamantanone to lactones. For cyclopentanone, cyclohexanone, cycloheptanone and cyclododecanone, the selectivity for lactones was very poor, but selectivity for acids was high.