The activity of water in {x(1)AlCl(3) + (1 - x(1))H2O} has been determined by measurement of vapour pressures in the temperature range (300 to 342) K, and the molality range (0.421 to 3.033) mol.kg(-1). The behaviour of the solution is described by the Pitzer model. The [Al(OH)(H2O)(5)](2+) in the system is calculated by considering the first hydrolysis step in the aluminium chloride solutions.