The hydrogenation of naphthalene in cyclohexane solvent was studied over a NiMo/Al2O3 catalyst operated in a fixed bed reactor. The effect of operating under sub- and supercritical conditions was investigated at temperatures 250-300 degrees C and pressures 20-90 bar. Naphthalene/cyclohexene flows of 0.5-2.5 mL min(-1) and hydrogen flows of 2-10 mL min(-1) were investigated. When operating in the supercritical region with high hydrogen/liquid feed ratio, no significant deactivation occurred over 6 h of operation. A strong effect of pressure upon conversion was observed, with naphthalene conversion close to 100% at 60 bar, 300 degrees C. Trans-decalin was the favoured product over most of the range of conditions investigated. When operating at low hydrogen/liquid feed ratio, initial conversion of naphthalene was 90-95%, declining to 58% at supercritical conditions and 35% at subcritical conditions after 7 h. Coke deposition was 38% lower at supercritical conditions. The ability of supercritical fluids to delay catalyst deactivation is considered, together with interesting changes to product distribution that occur under these conditions. (C) 2012 Elsevier B.V. All rights reserved.