Production of tertiary (hydroxymethyl)-methylphosphine oxides of the general formula: ##STR1## in which R stands for a branched or unbranched, substituted or unsubstituted alkyl, cycloalkyl, aralkyl or aryl group having from 1 to 18 carbon atoms. To this end tertiary bis-(hydroxymethyl)-phosphines of the general formula ##STR2## in which R has the meaning given above, are subjected to a thermal rearrangement reaction at temperatures higher than about 90.degree. C or at temperatures within the range -10.degree. and +250.degree. C, in the presence of a catalyst.