Infrared studies have demonstrated that decamethonium iodide dihydrate, bromide dihydrate, and chloride dihydrate, all of which contain planar C 2 h (H 2 O X - ) 2 clusters, are apparently isomorphous, as are anhydrous decamethonium iodide and bromide. Contrary to our previous report, decamethonium chloride has a structure different from the iodide and bromide. Since lattice enthalpy changes should be similar in the dehydration of the iodide and bromide dihydrates, the difference in the dissociation enthalpy should directly reflect differences in HOH X - hydrogen bond strength. The value of 5.15 kcal mol - 1 per HOH I - hydrogen bond broken in dehydration of the iodide dihydrate is too high relative to the other dihydrates to be accounted for by electronegativity, proton affinity, or ionic potential considerations. Vapor pressure vs. H 2 O content studies demonstrate that the dihydrate is the only hydrate formed by decamethonium iodide and bromide. The infrared spectrum of decamethonium chloride containing 1 mole H 2 O shows a mixture of the dihydrate and anhydrous material; thus the chloride also only forms a dihydrate.