1-Butene was isomerized to 2-butenes with a cis:trans ratio of similar to 2:1 over Pd/TiO2 under irradiation of UV light at room temperature, suggesting that a Pd/TiO2 photocatalyst can function as an effective substitute for a Lewis acid catalyst and that the reaction rate can be controlled by photon flux as in the case of changing temperature in a thermocatalytic system.