Zeolite-supported Pt and Pd catalysts were used to effect naphthalene hydrogenation with high trans/cis decahydronaphthalene (DeHN) isomer selectivity. Four zeolites were studied : three hydrogen-mordenites (HH) with different SiO2/Al2O3 ratios (17, 21 and 38) and HY (SiO2/Al2O3 of 5.0). Catalytic selectivity was found to depend on both the metal and the zeolite. Consequently, the catalyst composition could be tailored for selective production of either cis-DeHN or trans-DeHN. For example, Pt/HY showed especially high selectivity (80%) for cis-DeHN and unlike all other catalysts-tested, did not promote the isomerization of cis-DeHN. On the other hand, equilibrium between the DeHN isomers was achieved with Pd/HM21, giving ca. 93% trans-DeHN at 200 degrees C. In general, Pd catalysts showed higher initial selectivity for trans-DeHN than Pt catalysts. Also, the isomerization of cis-DeHN to trans-DeHN was more rapid on Pd catalysts. Selectivity for trans-DeHN increased with the fraction of weak acid sites on the zeolite (measured gravimetrically, using TPD of n-butylamine). No correlation between metal crystallite sizes (determined from XRD line-broadening) and DeHN isomer selectivity was found.