Matrix isolation of the vapour species produced by the passage of S 4N 4 vapour over silver selenide heated to ca 160°C produces unequivocal IR evidence (including the results of experiments with 15N-enriched samples) not of the thiazyl monomer, NS but of thionylimide, HNSO, in addition to disulphur dinitride, S 2N 2. The significance of these findings is considered in relation to possible synthetic routes to poly(sulphur nitride).