An equation representing photocatalytic degradation rate of three phenoxyacetic acid and two carbamate pesticides was proposed, which includes substrate adsorbability to TiO2 and Hammett's constant of substituent groups. The data fitting to this equation indicated that the adsorption was a predominant factor in determining the degradation rate. A study was also conducted on the degradation process of 4-chloro-2-methylphenoxyacetic acid (MCP) by identifying aromatic (4-chloro-o-cresol, 4-chlorocatechol and methylhydroquinone) and aliphatic acid (acetic, formic, glyoxylic and glycolic acids) intermediates. Particularly acetic acid formation was related to methyl substituent on aromatic ring.