Sterically hindered phosphoramidates such as N,N'-bis[di-(2,6-xylenoxy)phosphinyl]piperazine are prepared by the reaction of a sterically hindered diaryl chlorophosphate, such as di-(2,6-xylyl) chlorophosphate, with a basic nitrogen compound containing at least two basic N-H groups, preferably a heterocyclic compound such as piperazine, in a predominantly liquid, solvent-free reaction mixture. The reaction may be conducted in the melt or in the presence of a non-solvent carrier, typically an aliphatic hydrocarbon. An excess of the basic nitrogen compound is employed as an acid acceptor.