The ambient solubility of the mineral salts NaCl, KCl, CsCl, KBr, K2SO4 and CuSO4 is reported as a function of composition of mixed binary solvents consisting of water with polyethylene glycol (PEG 200), ethoxylated C9–11 alcohols (C10E6) and ethoxylated, or propoxylated C10–12 alcohols (C11E6P1). Solubility gradually decreases with decreasing water content and follows the order PEG 200 > C10E6 > C11E6P1. Solubility of CsCl and KBr was found to be surprisingly high in neat PEG 200, on the order of 1 mol kg−1, and in neat C10E6, on the order of 0.1 mol · kg−1. The observed solubility trends are explained by the theory of hard and soft acids and bases under the consideration of the polarity of the surfactants.