The transformation of cyclopentanone in the presence of H2S/H-2 was investigated at atmospheric pressure over a sulfided CoMo/Al2O3 catalyst. The main reaction products were cyclopentanethiol and cyclopentene, the relative amounts of which depended on the reaction temperature and on the H2S to cyclopentanone molar ratio. The best results were obtained at 220 degrees C, with a 2.5 H2S to cyclopentanone molar ratio : under these conditions, the cyclopentanethiol molar selectivity remained at about 90%, in a range of cyclopentanone conversion of 10-70%.