N.m.r. parameters (1H, 11B, 13C, and 31P) of a triazaphosphadiborine and a triazaphosphaborolidine and their corresponding ‘phosphinylium’ cations have been measured. The geminal 31P–13C coupling constants are consistent with non-planarity of the ring systems, this non-planarity being greater in the neutral than in the cationic species. Values of 3J(31PNCH) are less sensitive to changes in ring conformation