The present invention relates to phosphorus-containing compounds based on 1-hydroxypropane-1,3-diphosphonic acid and to the acid itself of general formula (I) ##STR1## in which M.sup.1, M.sup.2, M.sup.3, M.sup.4 and M.sup.5, independently of one another, each denote a radical from the series hydrogen, alkali metal, an equivalent of ammonium, alkyl- or hydroxyalkyl-substituted ammonium,a process for the preparation thereof, novel intermediates and the use thereof as a corrosion inhibitor, scale inhibitor and as a flame retardant.