Abstract: AIM　To study the effect of solution pH value on the chelation structure of zinc acexamate. METHODS　A series of samples at different solution pH values were prepared by 10% HCl or 1 mol.L^-1 NH3.H2O. Then API/TOFMS with electrospray ion source was applied to assay the samples. The nitrogen curtain gas and nebulizer gas were adjusted to a constant flow rate of 0.6 μL.min^-1 and 2 μL.min^-1 respectively. Samples were infused into the electrospray interface using a 500 μL syringe pump at a flow rate of 5 μL.min^-1. Mass spectra were acquired in positive ion modes by scanning over the range of m/z 100～1 000. RESULTS The chelation structure of zinc acexamate is stable at pH 2.54 and it can be easy to form the ion (M+ZnY)⁺ (Y=CH₃CONH(CH₂)₅COO^-) and (2M+Na)⁺ in this condition. CONCLUSION　The drug is an effective antiulcer agent. It may decrease the acidity of stomach juice, and form a polymer to protect the ulcer.