Objective To investigate the toxicity of soman and sarin to butyrylcholinesterase and to compare the intensity of their inhibitory effect on butyrylcholinesterase. Methods k m value of the duck serum butyrylcholinesterase was detected by modified Ellman method. The dissociation (k d), phosphonylation (k 2), and biomolecular reaction (k i) constants were determined for the inhibitory kinetics of soman and sarin. Results Under the experiment conditions, the k m value of cholinesterase was 1.44×10 -5 mol/L. The k d values of soman and sarin to cholinesterase were 0.88×10 -8 mol/L and 1.87×10 -8 mol/L. The k 2 values were 9.89/min and 11.70/min. Conclusion Although there is little difference between the reactive velocities of soman and sarin when the reversible enzyme inhibitor complex is transformed to covalence phosphonylated enzyme, the affinity and inhibitory capacity of soman are higher than those of sarin. This may be the reason why the toxicity of soman is higher than that of sarin.