The ring-opening mechanism influencing effect of a major product in the cyclohexene oxide-D-2 system was investigated over a Cu/SiO2 catalyst. This product is cyclohexene, thus, the hydrogenative ring opening of a 1:1 cyclohexene oxide-cyclohexene mixture was studied in the presence of D-2 at 403 K in a closed circulation reactor. It was found that the mechanism of single C-O scission was not affected, but that of the double C-O scission was changed. Simultaneous bond cleavage was the major route of ring opening in the additive-free system and it became consecutive on cyclohexene addition. Added cyclohexene was hydrogenated with a very low rate, but it transformed the surface of the catalyst and, thus, facilitated the change in the mechanism. An explanation concerning the seemingly anomalous lack of deuterium in a product (cyclohexane) not seen in the additive-free system is also suggested.