Ionization constants for the metal-bound H2O in sperm whale ferrimyoglobin, and in the corrinoids: factor B, factor V1A, and aquocobalamin, have been determined over a range of ionic strength and temperature. The following thermodynamic parameters at 25·0 °C and I= 0 were obtained (ΔH° in kcal. mole–1; ΔS° in cal. mole–1deg.–1; values ± mean uncertainty): sperm whale ferrimyoglobin, pK°= 8·94 ± 0·015, ΔH°= 6·20 ± 0·15, ΔS°=–20 ± 1; factor B, pK°= 10·94 ± 0·01, ΔH°= 9·20 ± 0·50, ΔS°=–19 ± 2; factor V1A, pK°= 11·01 ± 0·02, ΔH°= 6·18 ± 0·50, ΔS°=–30 ± 2; aquocobalamin, pK°= 7·64± 0·02, ΔH°= 4·60 ± 0·50, ΔS°=–19 ± 2. The results show that the thermodynamics of ionization are determined by the cumulative effect of several factors including the change on the metal, the nature of the trans-ligand, the extent to which H2O is involved in hydrogen bonding, and by the electrostatic environment of the molecule as a whole.