The dimerization of conjugated cyclic olefins during hydrogenation at low temperatures (<= 250 degrees C) on a spent commercial Ni-Mo-S/gamma-Al2O3 catalyst is reported. Hydrogenation of 4-methylstyrene versus alpha-methylstyrene showed that the methyl group attached to the vinyl group of alpha-methylstyrene decreased the dimer yield as a result of steric hindrance, while the yield of hydrogenated products remained high. The addition of 20 wt % cyclohexene to 4-methylstyrene and reaction at a lower temperature (200 versus 250 degrees C) decreased the 4-methylstyrene hydrogenation rate. An increased concentration of 4-methylstyrene and a lower reaction temperature increased dimer and gum yields. The data indicate that dimers are precursors to gum formation and that catalyst deactivation is linked to gum formation that results in an increased carbon content and a decreased Brunauer Emmett Teller surface area of the used catalyst. Furthermore, an increase in pressure drop across the fixed-bed reactor with time on stream (TOS) observed with 4-methylstyrene as the reactant but not with alpha-methylstyrene is consistent with cumulative gum deposition in the catalyst bed. The pressure drop is well-described by the Ergun equation, assuming that gum deposition reduces bed voidage with TOS.