Diphenylnitrilimine adds to the phosphazene bond of the azaphosphole 1. A single‐crystal X‐ray investigation of the cycloadduct 3 shows a planar bicyclo[3.3.0]octane skeleton meridional to the trigonal bipyramidal phosphorus. The structural data are compared to those of the starting compound 1, to those of the trifluoroacetone adduct 2, and to the change from ideal tetrahedral to ideal tbp geometry.© 1999 John Wiley & Sons, Inc. Heteroatom Chem 10:243–245, 1999