The invention relates to a process for the preparation of phosphorus-containing compounds of the formula (I) ##STR1## where R.sup.1 and R.sup.2 are identical or different and are a straight-chain or branched alkyl radical having 1 to 8 carbon atoms, a cyclohexyl-, cyclopentyl-, aryl-, halogen-, (C.sub.1 -C.sub.6)-alkyl- or (C.sub.1 -C.sub.6)-alkoxy-substituted aryl radical, where R.sup.1 and R.sup.2, together with the phosphorus atom and the oxygen atom, can also form a ring, in particular an oxaphosphorin ring, which comprises metering acrylonitrile at elevated temperature into a compound of the formula (II) ##STR2## where R.sup.1 and R.sup.2 are as defined above.