Magnesium dialkyl phosphates capable of thickening or gelling nonpolar organic solvents are prepared by reacting a dialkyl acid phosphate with a magnesium oxide, hydroxide, or carbonate in organic solvent solution in the presence of a small amount of water within defined limits of time, temperature, mole ratios of reactants, and size of alkyl groups. The products are useful as thickening agents and vapor pressure depressants for nonpolar solvents.