The hydrolytic reactions of a nickel ion in an ethanol-water mixture system were studied by using titration techniques and by considering the nickel nitrate complexes in the system. It is shown that Ni-4(OH)(4)(4+) is the main species in 21.2 and 44.4 wt % ethanol-in-water solutions between 25 and 15 degrees C. The values of the formation constant Q(4,4) for Ni-4(OH)(4)(4+) and the formation constants for the nickel nitrate complexes (beta(1) and beta(2)) are obtained by fitting the experimental data to equilibrium models. The solvating reaction of a nickel ion in ethanol-water solutions is included in the analysis of data and the equilibrium constant (alpha) is found to be a number smaller than 10(-9). It is shown that the precipitation pH of the Ni(OH)(2) will decrease as the ethanol content increases and as the temperature of the solution increases. The values of log Q(4,4) change from -26.3 +/- 0.10 to -20.4 +/- 0.06 when the temperature changes from 25 to 75 degrees C in 1.0 M Ni(NO3)(2) aqueous solutions. The values of log Q(4,4) change from -23.9 +/- 0.2 to -19.6 +/- 0.1 in a 1.0 M Ni(NO3)(2), 44.4 wt % ethanol solution over the same temperature range. At a fixed temperature, the values of Q(4,4) increase as the concentration increases and this is explained by the effect of ionic strength in the solution. The results also show that the standard enthalpy changes (Delta H degrees) for the hydrolytic reaction of Ni-4(OH)(4)(4+) were 183.9 +/-4.5 and 167.2 +/- 9.6 kJ/mol in 21.2 and 44.4 wt % ethanol-in-water solutions.