Electrochemical enhancement of the photocatalytic decomposition of isopropyl alcohol in gas phase was carried out in a polymer electrolyte cell. An external electrical field was created in the gas-phase photocatalytic reaction due to the presence of solid electrolyte. The effects of bias potential, relative humidity, initial isopropyl alcohol concentration and UV light intensities were investigated for the photoelectrocatalytic process. Photoelectrocatalytic decomposition of isopropyl alcohol was observed to be less sensitive to humidity for experiments that applied a bias potential higher than 2.5 V. Electrical current densities of photoanode were highly correlated to the variations in bias potential, relative humidity level and UV light intensity, although independent of inlet isopropyl alcohol concentrations. Humidity had an ambiguous effect on the membrane conductivity of polymer electrolyte and photocatalytic decomposition rate of isopropyl alcohol with the application of bias potential. The experimental results also showed that the energy consumed in the photoelectrocatalytic process was much less than that needed by the photocatalytic process to achieve a similar level of isopropyl alcohol decomposition.