During investigations of 1-hexene isomerization on a Pt/gamma-Al2O3 catalyst, hexene oligomers are found to form in the fluid phase. The total amount of oligomers increased by roughly an order of magnitude with isothermal increases in pressure from subcritical to supercritical values. These oligomers are significant coke precursors, increasing the coke formation and catalyst deactivation rates. Dilution of the hexene feed with an inert cosolvent like n-pentane, such that the reaction mixture is at supercritical conditions, reduces oligomer concentrations while maintaining the in situ extraction of the coke-forming compounds. Consequently, coke laydown is significantly reduced resulting in increased isomerization rates and decreased deactivation rates. Although oligomer formation and coke laydown decrease continuously with cosolvent addition, there exists an optimum cosolvent fraction beyond which the isomerization rate becomes limited by 1-hexene concentration and hence decreases. It was determined that ppm levels of organic peroxides in the hexene feed, formed as a result of exposure to air during either storage or the experiments, aid the formation of hexene oligomers in the fluid phase. Pretreatment of the hexene feed with activated alumina results in the virtual elimination of the peroxides and a severalfold decrease in total oligomer concentration. Our results indicate that peroxide removal from the hexene feed and cosolvent addition can significantly improve isomerization rates, catalyst activity maintenance, and pore accessibility in supercritical reaction mixtures.