The adducts of complex inorganic acids (CIAs) with monosubstituted derivatives of polyethylene glycols (PEGs) were prepared in aqueous acids. These adducts are poorly soluble in water (below 1 g/l) and readily soluble in polar organic solvents (up to 400 g/l). The adduct stoichiometry is a function of acidity, and the physicochemical and extraction characteristics of adducts depend on particular PEG and CIA. With increasing PEG content in the adducts, their extractive power decreases, the solubility in polar organic solvents increases, and the solubility in water passes through a minimum. All these adducts are highly resistant to hydrolysis and radiation. The extraction of Cs, Sr, Ba, Eu, and Am from aqueous HNO₃ with solutions of phosphomolybdic and hexachloroantimonic acid adducts with oxyethylated n-nonylphenol in organic solvents was studied.