Catalytic activity of iron complex-encapsulated zeolite catalyst ([Fe(bpy)(3)](2+)@Y) for oxidation of benzene increased with increasing the amount of water added to acetonitrile. Maximum value of catalytic activity was obtained when the volume ratio of the solvents was equal (CH3CN:H2O = 1:1). After reaction at 24 h, the turnover number of [Fe(bpy)(3)](2+)@Y was higher than that of [Fe(bpy)(3)](ClO4)(2) as a homogeneous catalyst.