AbstractAb initio theoretical calculations were carried out on the three tautomers, NH, OH and CH, of 1,2,6‐thiadiazine 1,1‐dioxides. Different basis sets were employed in order to obtain an adequate description of these cyclic sulphamide derivatives. A coherent picture is obtained which includes relative stability of tautomers (NH > CH > OH), non‐planarity of NH and CH tautomers and electronic distribution (Boys' localized orbitals).