The He(I) photoelectron spectrum of nitrosomethane has been re-examined and re-assigned on the basis of ab initio SCF–CI calculations for the molecule and the three lowest doublet ion states and a comparison with isoelectronic molecules. He(I) and in most cases He(II) photoelectron spectra and assignments are presented for t-nitrosobutane(2-methyl-2-nitrosopropane), trifluoro-, trichloro- and tribromonitrosomethane, as well as for the mixed substituted chlorofluoro- and chloro-bromonitrosomethanes. The ionisation sequence n_– < n₊ < π(NO) implying a breakdown of Koopmans' approximation has been established for these compounds and n₊/n_– lone-pair splittings varying from 2.8 to 6.0 eV have been determined for the nitroso group. The characteristic ionisation energies associated with this group correlate well with the substituent electronegativities. For the trihalogenonitrosomethanes the halogen lone-pair ionisations have also been assigned by comparison with related molecules.