Abstract: The invention relates to a process for preparing phosphorus-containing compounds of the formula I ##STR1## in which R.sup.1 and R.sup.2 are an alkyl radical having 1 to 8 carbon atoms, a cyclohexyl radical, a cyclopentyl radical, an aryl radical or a halogen-, alkyl- or alkoxy-substituted aryl radical, where R.sup.1 and R.sup.2 together with the phosphorus atom can also form a ring, n and m=0 or 1 and R.sup.3 and R.sup.4 are alkyl radicals having 1 to 4 carbon atoms or n=0, m=0 or 1 and R.sup.3 and R.sup.4 are H, by adding compounds of the formula II ##STR2## into dialkyl esters of fumaric acid or of maleic acid of the formula (III) or dialkyl esters of itaconic acid of the formula (IV) ##STR3## in which R.sup.3 and R.sup.4 have the meaning given above, and, optionally reacting an alkyl ester of a phosphorus-containing dicarboxylic acid of the formula (I), where n=0, with water in the presence of catalytic amounts of an acid to give the phosphorus-containing dicarboxylic acid.