A proton magnetic resonance coordination number and ion-pairing study of GaCl3, GaBr3, GaI3, Ga(NO3)3and Ga(ClO4)3in acetone-water mixtures has been completed. By cooling solutions of these salts to about -50°c, proton exchange is slowed and separate resonance signals are observed for bulk water molecules and water molecules in the Ga3+solvation shell. The integration of these signals provides an accurate measure of the cation hydration number. In the case of Ga(NO3)3and Ga(ClO4)3, Ga3+is hydrated by six water molecules even at extremely high acetone concentrations. Also, only one signal is observed for the complexed water molecules. In contrast to this result, the hydration number of Ga3+decreases from 6 to about 1·4 for GaCl3, 1·6 for GaBr3and 2·5 for GaI3as large amounts of acetone are added to aqueous solution, and more than one complexed water signal is observed in the spectra arising from the GaCl3solutions of low water content. In addition to providing evidence for contact ion-pairing, the results are also indicative of octahedral and tetrahedral complexes of Ga3+.