The structure of N-methyltrichlorophosphinimine dimer, (CH₃NPCl₃)₂, has been determined by three-dimensional Fourier synthesis and full-matrix least-squares refinement of X-ray diffraction data. The structure is composed of discrete centrosymmetric dimeric molecules with a planar four-membered ring of alternate phosphorus and nitrogen atoms (P–N–P = 99·5°± 0·4° and N–P–N = 80·5°± 0·4°). The co-ordination of each phosphorus atom is approximately a trigonal bipyramid of three chlorine atoms and two nitrogen atoms, one apical and one equatorial. The apical bond distances are P–Cl = 2·133 ± 0·003 Å and P–N = 1·769 ± 0·007 Å; the equatorial are P–Cl = 2·026 ± 0·004 Å and P–N = 1·635 ± 0·007 Å. The co-ordination of the nitrogen is trigonal with a N–C bond distance of 1·475 ± 0·010 Å. The shortness of the P–N bonds and the planar co-ordination of the nitrogen atoms suggest some dπ–pπ bonding between the phosphorus and nitrogen atoms.