The isomerization and oligomerization of 1-hexene was studied over ZSM-5 catalyst under sub- and supercritical conditions within a down-flow fixed bed reactor in the temperature range of 220−250 °C and pressure range 10−70 bar. The catalyst activity and the product selectivity were found to be dependent on the operation conditions. Reactions were carried out over beds with 10 and 0.5 g catalyst, the smaller bed being used to simulate coking in the top layer of the larger bed. The conversion of 1-hexene over the 10 g bed of catalyst was in the range 83−99%, depending upon the operating conditions and was stable during the test period. An increase of the reaction temperature from 220−250 °C led to higher selectivity toward oligomerization, as did increases in reaction pressure in the range 10−70 bar. The amount of coke deposited on the catalysts decreased from 18.8 wt % at 235 °C and 10 bar in the subcritical region to 10.4 wt % at 235 °C and 40 bar in the supercritical region. DRIFTS showed that deposited coke is mainly polyolefinic. Nitrogen sorption showed that following initial shallow pore filling over shorter contact times, the pore mouth subsequently became blocked and coke mainly formed on the outside of the zeolite crystallites. When operating with 0.5 g catalyst, some deactivation occurred under subcritical conditions, although not under supercritical conditions. It appeared that with the smaller catalyst mass oligomers were retained within the pore structure of ZSM-5. The results of the study demonstrate that adjustment of the operating temperature and pressure can be used to tune the product selectivity of the reaction and the total amount of coke deposited upon the catalyst is reduced by operating under supercritical conditions.