The FeII of the binuclear FeIIFeIII active site of pig purple acid phosphatase (uteroferrin) has been replaced in turn by five MII ions (MnII, CoII, NiII, CuII, and ZnII). An uptake of 1 equiv of MII is observed in all cases except that of CuII, when a second more loosely bound CuII is removed by treatment with edta. The products have been characterized by different analytical procedures and by UV-vis spectrophotometry. At 25 °C, I ) 0.100 M (NaCl), the nonenzymatic reactions with H2PO4 - give the í-phosphato product, and formation constants K/M-1 show an 8-fold spread at pH 4.9 of 740 (Mn), 165 (Fe), 190 (Co), 90 (Ni), 800 (Cu), 380 (Zn). The variations in K correlate well with stability constants for the complexing of H2PO4 - and (CH3O)HPO3 - with MII hexaaqua ions. At pH 4.9 with [H2PO4 -] g 3.5 mM rate constants kobs decrease, and an inhibition process in which a second [H2PO4 -] coordinates to the dinuclear center is proposed. The mechanism considered accounts for most but not all of the features displayed. Thus K1 values for the coordination of phosphate to MII are in the range10-60 M-1, whereas K2 values for the bridging of the phosphate to FeIII are in the narrower range 7.8-12.4. From the fits described Ki 103 M-1 for the inhibition step, which is independent of the identity of MII. Values of kobs decrease with increasing pH, giving pKa values which are close to 3.8 and independent of MII (FeII, ZnII, MnII). The acid dissociation process is assigned to FeIII-OH2 to FeIII-OH-, where OH- is less readily displaced by phosphate.