The pressure-tunability of the diffusivity of a supercritical medium in a mesoporous catalyst, by approximately 2 orders of magnitude with moderate changes in pressure, is demonstrated. Effective diffusivities in porous Pt/gamma-Al2O3 pellets at supercritical. conditions were estimated by performing the geometric isomerization of 1-hexene (T-c = 231 degreesC, P-c = 31.7 bar) at supercritical conditions that encompass both the kinetic-controlled and the pore-diffusion-controlled regimes. Operation at supercritical conditions thwarts catalyst deactivation by coking and ensures steady catalyst activity. Effective rate constants (etak) are calculated from steady-state 1-hexene conversion data obtained in a fixed-bed reactor at various supercritical pressures (37-73 bar) in the 235-310 degreesC range. The intrinsic kinetics parameters and the effective diffusivities are estimated from the rate constants (etak) using the conventional theory of diffusion and reaction in catalyst pellets. The intrinsic activation energy was determined to be 184 +/- 4.4 kJ/mol. The pore diffusivity of the hexene molecules was found to vary by nearly 2 orders of magnitude from 6.3 (10(-6)) cm(2) s(-1) at rho(r) (reduced density) = 2.7 (235 degreesC, 72.8 bar) to 1.4 (10(-4)) cm(2) s(-1) at rho(r) = 0.8 (311 degreesC, 36.7 bar), with relatively moderate changes in pressure and temperature of the near-critical reaction mixture.