The densities of the salt hydrate phase-change materials (PCMs) crystalline ammonium alum (aluminum ammonium disulfate dodecahydrate), aluminum nitrate nonahydrate, and magnesium nitrate hexahydrate as well as their melts and of solid aluminum sulfate and magnesium acetate hydrates and anhydrous ammonium nitrate and sulfate were determined as a function of the temperature in the solid and liquid phases. The densities of selected mixtures of these salts were also determined as a function of the temperature. Viscosities of the corresponding molten single salts and mixtures were also determined.