The competitive transformation of cyclopentanone and of two thiols (cyclopentanethiol and cyclohexanethiol) was carried out on a sulfided CoMo/Al2O3 catalyst at 220 degreesC under atmospheric pressure in the presence of H-2/H2S (90/10 vol.%). The results made it possible to explain the good selectivity in cyclopentanethiol obtained in the thioreduction of cyclopentanone. This is the consequence of the inhibition due to a competition to adsorption, of the thiol desulfurization into the corresponding olefin. The kinetic order with respect to cyclopentanone of the transformation of cyclohexanethiol into cyclohexene was found equal to -0.6.