The crystal structure and the electron density of the benzene solvate of hexaaziridinyldyclotriphosphazene have been analyzed using single-crystal X-ray diffraction at 200 K with Mo K alpha radiation and an area detector to a resolution of sin(theta(max)/lambda = 1.25 Angstrom(-1); 17 639 measured reflections gave 4596 unique reflections with R, = 0.027. A multipole atomic density model was fitted to 4038 reflections with I > 3 sigma(I) to give an R(F-2) = 0.039. The dynamic deformation densities of the trapped benzene molecule, the two unique three-membered aziridinyl rings, and the phosphazene ring have all been examined and are reported here for the first time. Those of the benzene and the aziridinyl groups conform to the expected density for these systems. These observations are used to validate the observations of the dynamic deformation density in the phosphazene ring. The observed dynamic deformation density in this ring corresponds to the density for the pi/pi’ models with conjugation from a phosphorus atom, through a ring nitrogen atom to a second phosphorus, but with nodes at each of the phosphorus atoms. Theoretical dynamic deformation densities, derived from ab initio calculations and carried out on the simple aziridinyl and phosphazene rings, closely resemble the observed deformation densities.