The effect of α- and β-cyclodextrin-Cu(II) (1 : 2) complexes on the hydrolysis of phosphorothionate esters was examined in 1.0 M NaOH solution at 25℃. The β-cyclodextrin-Cu(II) complex remarkably decelerated the hydrolysis of fenitrothion, methyl parathion, and parathion. The corresponding α-cyclodextrin complex also remarkably decelerated the hydrolysis of methyl parathion and parathion, although it slightly accelerated the reaction of fenitrothion. Each reaction proceeds via the prior formation of an adduct between the phosphorothionate ester and the complex, followed by the nucleophilic attack of the hydroxide ion on the reaction site of the ester included within the void space of the complex. The deceleration of the reaction may be caused by the retardation of the nucleophilic attack of the hydroxide ion by the cyclodextrin-ring in the complex. On the basis of the observed equilibrium and kinetic parameters involved in the present reaction mechanism, the orientation of the esters within the complex cavity was discussed.