We studied the hydrolysis and TiO2 photocatalysis of the pesticide dichlorvos at pH 2.6-9 using phosphorus-31 nuclear magnetic resonance (31P NMR). We found that the hydrolysis of dichlorvos led to the formation of dimethyl phosphate at pH 5.6-9. On the other hand, TiO2 photocatalysis decomposed dichlorvos into dimethyl phosphate, which further reacted to form monomethyl phosphate at pH 2.6 and 9. Monomethylphosphate was hydrolysed into phosphate at pH 2.6. P-31 NMR is therefore a promising tool to study the degradation of organophosphorus pesticides.