During 1-hexene (P-c = 31.7 bar; T-c = 231 degrees C) isomerization on a Pt/gamma-Al2O3 reforming catalyst, the formation of hexene oligomers in the bulk fluid causes catalyst deactivation with time. Organic peroxides present in the 1-hexene feed stock are responsible for the formation of these oligomers. With 130 ppm peroxides in the feed, the catalyst undergoes total deactivation in about 10 hours at subcritical conditions (281 degrees C, 1 bar and 135 g hexene/h/g cat.). The deactivation rate is reduced at supercritical operating conditions due to the ability of supercritical reaction media to solubilize and extract these oligomers from the catalyst surface. When the feed peroxides are virtually eliminated prior to introduction into the reactor (by pretreatment with activated alumina and by simultaneous deoxygenation of the feed), nearly constant isomerization activity is observed with about 45% hexene conversion to isomers at subcritical conditions, and about 65% conversion at supercritical conditions (281 degrees C, 70 bar). During an extended 42 hour run at supercritical conditions, catalyst activity was nearly constant with neither measurable coke laydown nor surface area/pore volume losses in the spent catalyst.