Abstract: Described are mixed boric acid/phosphoric acid esters which may be obtained by subjecting at least one phosphorus compound capable of ester formation and at least one boron compound capable of ester formation simultaneously to an esterification reaction with at least one polyol and/or a derivative thereof capable of ester formation. Such esters may be prepared in an aqueous medium and find use in the flame-proofing treatment of a variety of materials, as acid catalysts for acid-curing synthetic materials, for controlling boron deficiencies in cultivated plants, and in many other fields of application.