SeCl₄ reacts with Ph₃PNSiMe₃(2) by elimination of SiMe₃Cl to form Ph₃PNSeCl₃(3), which is converted to (Ph₃PN)₂SeCl₂(4) by a further mole of (2). Compound (4) may also be prepared from (2) and SeOCl₂, or Se₂Cl₂; (4) reacts with SbCl₅ to form [(Ph₃PN)₂SeCl]⁺[SbCl₆]^–(5). 2,4,6-Tri-t-butylaniline and SeOCl₂ give chloro-N-(4,6-di-t-butyl-2-oxocyclohexa-3,5-dien-1-ylidene)selenamide, (7). Compounds (3),(4),(5), and (7), the previously reported diphenyl-N-(p-toluenesulphonyl)selenimide, (1), and 5,6-dimethyl-2,1,3-benzoselenadiazole, (8), have been studied by ⁷⁷Se n.m.r.; the crystal structures of (1), (3), (4), and (7) have been determined by X-ray diffraction. The Se–N bond lengths of 1.787(6), 1.680(4), 1.735(4), and 1.766(4)Å all indicate significant Se–N multiple bonding when compared with Se–N bonds from the literature, but curiously the longest of the four is in (1), the only structure containing a formal double bond. Although bond length alternation indicates that the quinone type resonance extreme predominates for (7), the Se–N and Se–O distances are shorter than in an analogous compound, N-(3,5-dimethyl-2-oxocyclohexa-3,5-dien-1-ylidene)phenylselenamide, the Se–O distance being reduced by 0.5 Å to 2.079(3)Å.