The present study includes measurements on the solubility, density, and refractive index of saturated ferric sulphate-arsenic oxide-water solutions at 323.15 and 343.15 K for different mass fractions of arsenic and iron. The phase diagram was determined at 323.15 and 343.15 K and its regions were defined. The most important salt precipitated at both temperatures was Fe(H(2)ASO(4))(3)center dot 5H(2)O (kaatialaite). Equations are proposed which correlate the equilibrium and physical property data of the ternary systems as a function of the mass fraction of iron. These equations allowed determination of invariant points (double salt saturations) for both temperatures studied. Finally, the solubility of Fe(H(2)ASO(4))(3)center dot 5H(2)O in aqueous solution was determined in order to evaluate the feasibility of its use as a stable (non-leaching) waste disposal alternative. (c) 2007 Elsevier B.V. All rights reserved.