Kieserite (MgSO4 center dot H2O) solubility was measured at temperatures between (338 and 378) K, FeCl3 concentrations of 1.0 mol.kg(-1), MgCl2 Concentrations between (1.1 and 3) mol.kg(-1), and HCl concentrations between (0.5 and 2.3) mol.kg(-1). These conditions are of interest because of their relevance for processes designed to extract nickel from oxide ores by atmospheric leaching in mixed sulfate-chloride solutions. Experimental data were compared with model predictions generated by use of OLI Software's mixed solvent electrolyte (MSE) model. Model predictions for MgSO4 center dot H2O solubility were found to match experimental results with an average absolute deviation of +/- 0.09 mol.kg(-1) and an average absolute relative deviation of 11.1 %. The ability of the model to accurately predict solubility trends in solutions not used in the development of model parameters validates the model.