The water activities of the systems MSO4 + H2O (M = Mn, Co, Ni, Cu, Zn) at higher temperatures are important for heavy metal hydrometallurgy processing of these metals. This paper reports water activity data for these binary systems at 323.15 K from isopiestic measurements. The reliability of the apparatus was verified by determination of the water activities of the test systems at 298.15 K and also two reference systems CaCl2 + H2O and H2SO4 + H2O at 298.15 K and 323.15 K. The results obtained agree well with literature data, with maximum relative deviations of the water activities for the reference systems of less than 0.032 % at 298.15 K and 0.062 % at 323.15 K respectively. The water activities for the MSO4 + H2O (M = Co, Ni, Cu, Zn) systems at 323.15 K were found to be approximately the same up to high concentrations, but were significantly lower than the MnSO4 + H2O system. The relative deviation of isopiestic molality of parallel samples in each experimental run is better than 0.2 %. The measured water activities reported in this work can be used to parametrize thermodynamic models of heavy-metal hydrometallurgical processes.