The hydrogenation and dehydrogenation of cyclohexene has been studied in temperature-programmed and isothermal modes over oxide-supported Pt surfaces. The ratio of the rates of dehydrogenation:hydrogenation measured experimentally does not appear to correlate with that expected from standard thermodynamic free energies of formation. This modification of normal descriptions of catalysis could occur if reaction products separate and segregate on the Pt surface. Consideration is given to which sites may be predominantly responsible. Evidence of structure sensitivity is also highlighted.