In a process for the preparation of an ethane-1,2-diphosphinic acid diester of the formula ##EQU5## wherein R.sub.1 represents an alkyl, phenyl or phenalkyl group having from 1 to 18carbon atoms and R.sub.2 is an alkyl group or a halogen substituted alkyl group having from 1 to 18 carbon atoms by reacting with acetylene a phosphorous acid ester of the formula ##EQU6## wherein R.sub.1 and R.sub.2 a