Ab initio molecular orbital calculations were carried out to determine the structure and conformation of chlorosulfonylisocyanate and cyclopropylisocyanate. Geometrical parameters and harmonic vibrational wavenumbers calculated at the MP2/6-31G(d,p) level showed that while chlorosulfonylisocyanate has only a gauche equilibrium structure, cyclopropylisocyanate exhibits two stable conformations. The rotational barriers are rather small. These theoretical results contribute to a clarification of available experimental results. Rotational constants and vibrational wavenumbers have also been analysed.