Organophosphate pesticides are used in large quantities. Once accumulated in the environment, they exhibit toxic effects on non-target organisms. Dimethoate, a frequently used insecticide, and its oxo-analog omethoate inhibit acetylcholinesterase and are toxic for mammals. However, under environmental conditions, they also undergo chemical transformations and decomposition. Nevertheless, the systematic data about dimethoate and omethoate decomposition are missing. We performed a systematic analysis of dimethoate and omethoate decomposition under different pH conditions and estimated their long-term eco-neurotoxic effects. Dimethoate and omethoate decompose rapidly in alkaline aqueous solutions (half-lives 5.7 ± 1.4 and 0.89 ± 0.21 days) but are highly stable in acidic solutions (half-lives 124 ± 18 and 104 ± 9 days). These differences are explained using quantum chemical calculations, indicating that a weaker P–S bond in omethoate is more susceptible to hydrolysis, particularly at a high pH. The toxicity of these pesticide solutions decreases over time, indicating that no or very little of highly more toxic omethoate is formed during hydrolysis of dimethoate, pointing to the advantage of alkaline hydrolysis over other techniques for dimethoate removal. Presented data are used to benchmark dimethoate and omethoate concentrations and toxicity in contaminated water in the pH range 3 to 9 for up to 70 days upon the release in the environment. Graphical abstract