Vinyl chloride monomer (VCM, CH2CHCl) was photocatalytically mineralized on TiO2 (anatase and rutile) and ZnO at room temperature. The VCM decomposition rate was larger in the order of anatase > rutile > ZnO. VCM was decomposed into CO2, CO, HCl and long-lived intermediates (formic acid and monochloroacetyl chloride), which were further decomposed into CO2 and H2O; more than 99.5% of carbon atoms in VCM were changed to CO or CO2 by UV irradiation on TiO2, (P25) within 360 min. Formic acid was also formed in the photocatalytic decomposition of ethylene on the TiO2 pretreated with VCM. Cl- ions adsorbed on TiO2 participated in the decomposition of VCM or ethylene and enhanced the formation of formic acid and CO.