Calf intestinal alkaline phosphatase (EC.3.1.3.1) is a dimeric metalloenzyme composed of two identical subunits, the each active site of which contains a tight cluster of two zinc ions and one magnesium ion. The kinetic theory of the substrate reaction during irreversible inhibition of enzyme activity previously described by Tsou has been applied for a study on the kinetics of the course of inactivation of the enzyme by EDTA. The kinetics of the substrate reaction with different concentrations of the substrate p-nitrophenylphosphate (PNPP) and inactivator EDTA suggested a competitive complexing mechanism for inactivation by EDTA, and the process of inactivation composed of the rapid initial formation of an enzyme-EDTA complex, in which the conformation of enzyme has been changed, and then zinc ions are finally removed from the enzyme.