The complex (EDDA)Fe-III (H(2)EDDA = HOOCCH2NHCH2CH2NHCH2COOH) promotes hydrolysis of 4-nitrophenyl phosphate (NPP) to 4-nitrophenol(ate) (NP) and orthophosphate (P-i) in weakly acidic aqueous media (25 degrees C). Maximum rates are achieved at pH 6.0 under conditions where (EDDA)Fe-III is in considerable excess to NPP (25 degrees C). Up to 2:1 molar ratios of (EDDA)Fe-III to NPP, the hydrolysis is remarkably slow, but as soon as the ratio exceeds 2:1, the rate increases markedly. The results indicate that a 3:1 iron to NPP complex is required to achieve effective hydrolysis, which presumably proceeds through attack of the phosphorus by an iron(III)-coordinated hydroxide.