o-, m-, p-xylene and their tetrahydro derivatives were hydrogenated by a reduced nickel catalyst over the temperature range 150∼250°to investigate the mechanism of the stereoselective formation of cis and trans isomer in vapor phase. The selectivity for the formation of trans isomer was the greatest with p-xylene and the smallest with m-xylene. More than 50% trans isomer was produced with p- and o-xylene but not with m-xylene over the entire temperature range investigated. A similar result was obtained also for the hydrogenation of dimethylcyclohexenes. On the basis of these findings, the Horiuti-Polanyi-Siegel mechanism, as shown schematically in Fig. 6, was discussed taking into consideration various adsorbed types for the half-hydrogenated state. It was shown that the mechanism will meet some difficulties to account for the experimental results. On the other hand, the trans addition of hydrogen to π-allylic adsorbed species giving rise to trans isomer was considered to occur in particular at higher temperatures.