The effect of H2S on catalyst activity and product selectivity during hydrocracking of Fischer-Tropsch (FT) wax was studied using sulfided silica-alumina-supported NiMo and NiW catalysts. FT wax, which is sulfur free was hydrocracked at various H2S concentrations by addition of dimethyl disulfide (DMDS). Conversion and selectivity were affected by the reaction temperature and H2S concentration. On the basis of the classical hydrocracking mechanism, the observed variations in product selectivity were attributed to the change in the ratio of metal to acid sites, which was affected by H2S addition. Catalyst deactivation in commercial practice is compensated by increasing the operating temperature, but higher reaction temperatures create more unsulfided metal sites, which on the other hand result in catalyst selectivity changes. This effect has to be balanced by adjusting the H2S concentration accordingly.