A gas electron diffraction study results in a planar P3N3 ring with P-N = 1.583(3) Angstrom and endocyclic P-N-P and N-P-N angles close to 120 degrees. The geometry of the ring in the gas phase is identical to that in the recently reported crystal structure. Quantum chemical methods (HF/3-21G*, HF/6-31G*, and B3LYP/6-31G*) reproduce the gas-phase structure only reasonably well. [GRAPHICS]