Direct and indirect photolysis (λ ) 254 nm) of aqueous nicotine and phosphamidon were studied. A central composite design was used to explore the effects of initial [H 2O 2] (0-10 mM), pH (2.8-11.2), and ionic strength (I c = 0.02-0.71 M) on the rate constants of nicotine and phosphamidon separately dissolved in a surface water surrogate matrix. Five levels of each factor were included in the design. For nicotine, the fastest predicted photochemical oxidation rate constant (k= 0.43 min -1) occurred under the following conditions: pH ) 7.5, [H 2O 2] = 7.5 mM, and I c= 0.02 M. This rate constant predicts that 90% of the nicotine will react within 5.4 min under these conditions. In general, the photochemical oxidation of nicotine is more rapid at lower ionic strength and near-neutral pH values. For phosphamidon, the fastest predicted oxidation rate constant (k ) 0.65 min -1) occurred at a pH of 7.1 and [H 2O 2] of 5 mM. Under these conditions, 90% of the phosphamidon would react within 3.5 min of treatment. Like nicotine, the photochemical oxidation of phosphamidon is more rapid at near-neutral pH values. Ionic strength has no significant effect on the photochemical oxidation of phosphamidon. © 2009 American Chemical Society.