The cyclophosphamide metabolite phosphoramide mustard (PM) was synthesized with isotopic enrichment at each nitrogen and oxygen site. Sequential reaction of N,N-bis(2-chloroethyl)phosphoramidic dichloride [Cl2P(O)N(CH2CH2Cl)2] with benzyl alcohol and ammonia gave N,N-bis(2-chloroethyl)phosphorodiamidic acid phenylmethyl ester [BzO(H2N)P(O)N(CH2CH2Cl)2] . Catalytic hydrogenation of this benzyl ester followed by the addition of cyclohexylamine (CHA) provided PM as the CHA salt. Incorporation of (NH3)-N-15 into this general scheme gave PM with a (NH2)-N-15 moiety. Glycine-N-15 was converted to bis(2-chloro-ethyl)amine-N-15 hydrochloride which, in turn, provided for N,N-bis(2-chloroethyl)phosphoramidic-N-15 dichloride. Use of this compound in the general synthetic pathway yielded PM-CHA with N-15 in the mustard moiety. O-17-Enriched PM was generated through the use of benzyl alcohol-O-17. To obtain the alcohol, labelled benzaldehyde was made by exchange with (OH2)-O-17, and was then reduced with sodium borohydride.