The hydrodenitrogenation (HDN) of methylcyclohexylamine was studied over sulfided NiMo/gamma -Al2O3. 2-Methyl-cyclohexylamine (MCHA) is an intermediate in the HDN of toluidine that can be detected in addition to the main products, methylcyclohexene (MCHE) and methylcyclohexane (MCH), when a high quantity of cyclohexene (CHE) was added to a-toluidine during HDN. The selectivity of MCH in the HDN of MCHA was about 20% at zero conversion. The detection of 2-methylcyclohexanethiol (MCHT) in the HDN of MCHA explains why MCH is observed as a quasi primary product in the HDN of o-toluidine. MCH is formed via nucleophilic substitution of the amine group of MCHA, giving MCHT, followed by C-S bond hydrogenolysis. A kinetic investigation at different H2S partial pressures showed that H2S increases the rate of nucleophilic substitution, but leaves the rate of the beta -Hofmann elimination of ammonia from MCHA unchanged.