Heterogeneous photocatalysis using simulated solar irradiation in the presence of titanium dioxide was studied using dimethyl methylphosphonate (DMMP) as a surrogate for organophosphorus compounds including pesticides and chemical warfare agents. A reduction in solution pH accompanied DMMP degradation with the pH approaching the pKa of DMMP. Addition of peroxide before reaction increased the amount of DMMP degraded. Nearly complete destruction of DMMP was only achieved with the largest addition of peroxide. Photoreaction of DMMP in a pH 10 solution and a pH 10 carbonate buffer degraded a similar percent of the solute, with a more gradual reaction observed for the buffered solution. Variations in reaction conditions were studied to evaluate the effect of potential spill conditions. The presence of fulvic acid in the solution slightly enhanced the oxidation of DMMP and the absence of dissolved oxygen inhibited DMMP oxidation. DMMP solutions prepared in simulated sea water reached similar degradation percentages as pure solutions, however, reacted more slowly.