N-Phosphinonitrilimine 1 and phosphine azides 4 react with dimethyl acetylenedicarboxylate via formal [4 + 2] cycloadditions affording 1,2,3 lambda(5)-diazaphosphinine 2 (90% yield) and 1,2,3,4 lambda(5)-triazaphosphinines 5 (63-75% yield), respectively. Although 2 is reluctant toward dinitrogen extrusion, derivatives 5 afford 1,2 lambda(5)-azaphosphetes 6 (80-90% yield) by heating in refluxing toluene. Four-pi-electron four-membered rings 6 react via the ring nitrogen atom with a variety of electrophiles. The four-membered ring structure is preserved by addition of boron trifluoride or iodomethane, while ring-opening reactions are observed with water, pentafluorobenzonitrile, and carbon disulfide; ring expansion reactions occurred with dimethyl acetylenedicarboxylate, methyl isothiocyanate, phenyl isocyanate, trimethylsilyl isocyanate, and isothiocyanate.