The objective of this study was to establish the relative rate constants for the reactions of selected pesticides (linuron, diuron, prometon, terbacil, diazinon, dyfonate, terbufos, and disulfoton) listed on the U.S. EPA Contaminant Candidate Listwith UV and hydroxyl radicals (*OH). Batch experiments were conducted in phosphate buffered solution at pH 7. All pesticides were found to be very reactive toward *OH as indicated by rate constant values above 10(9) M(-1) s(-1). Using molinate as a reference compound, kOH ranged from 2.7 x 10(9) to 12.0 x 10(9) M(-1) s(-1) for the contaminants while slightly higher values from 2.9 x 10(9) to 14.3 x 10(9) M(-1) s(-1) were obtained using nitrobenzene as a reference compound. A method was established that accounts for direct photolysis when calculating kOH using UV/H2O2 process for compounds which degrade significantly by a direct photolysis mechanism.