Synthetic utilities of diethyl phosphorocyanidate (DEPC) are reviewed. Interaction of DEPC with carboxylie acids in the presence of triethylamine produces carboxyl-activated compounds such as acyl phosphates and/or acyl cyanides, which can easily couple with amines, thiols, alcohols, and active methylene compounds to give amides and peptides, thiol esters, carboxylic esters, and C-acylated products, respectively. Penicillin sulfoxides give 3-cephem as well as 3-methyienecepham derivatives by ring expansion with DEPC. DEPC can be also used as a cya-nation reagent for Ci-unit introduction to enamines and carbonyl compounds, the latter of which gives α-amino nitriles on treatment with DEPC and amines. Physiological properties of DEPC are also discussed. © 1979, The Society of Synthetic Organic Chemistry, Japan. All rights reserved.