Magnesium-dependent phosphatase models containing intramolecular NH...O hydrogen bonds were synthesized and structurally characterized by X-ray analysis. The Mg-O bond distances varied with the mode of the hydrogen bonds. H-1 NMR spectra in nonpolar solvents revealed that the acidity of the coordinated water molecule was regulated by the hydrogen bonds. Further, stoichiometric hydrolysis of phosphoric ester significantly depended on the hydrogen bonds. Zinc analogues showed similar but smaller dependencies, which suggest the indispensable role of Mg2+ ion in the activation of the enzymes.