Production of tertiary phosphine oxides of the general formula (I): ##STR1## in which the substituents R.sub.1 and R.sub.2, being identical or different, each stand for an alkyl, alkenyl, alkinyl, cycloalkyl, aralkyl or aryl radical having from 1 to 18 carbon atoms. The phosphine oxides are made by splitting off formaldehyde and hydrogen halide at temperatures within the range about 150.degree. and 350.degree. C, in the presence of hydrogen halide, from a molten compound of the general formula (II): ##STR2## in which R.sub.1 and R.sub.2 have the meanings given above and X stands for halogen; separating the formaldehyde and hydrogen split off from the reaction mixture, and treating the resulting crude product so as to separate the compound of general formula (I) therefrom.