Storage instability is one of the serious problems that greatly restrict pesticide use. Routine checks on the composition and toxicity of 30% emulsifiable concentrates (EC) of chloramidophos (CP) during storage indicated that 78.6% of the active ingredient had decreased, whereas the anti-acetylcholinesterase (AChE) activity of the formulation was potentiated by 3.5 times. To understand the mechanism for these changes, detailed knowledge of the products present and their effects on anti-AChE potential deserves attention. It was likely that the basis for these changes was methanol, the cosolvent of CP EC, because when the purified CP was stored in methanol at 50 degrees C for 2 weeks, CP drop and toxic potentiation similar to those observed in CP EC also appeared. The major products of the CP-methanol reaction mixture were isolated and identified by HPLC and GC-MS, respectively, and their inhibitory potentials against AChE and effectiveness as potentiators were assessed. Following redetermination of the main product (O,S-dimethyl-[(2,2,2)-trichloro-1-methoxyethyl]phosphoramidothioate (MCP)) and high anti-AChE material (methamidophos), which were preconfirmed in the reaction mixture in CP EC, it was successfully demonstrated that the majority of CP in the formulation had been transformed to a new stable compound, MCP. Meanwhile, formation of another product, methamidophos, resulted in toxic potentiation.