1H nmr (3JHH values) and 13C nmr results (shielding effect of vicinal methyl 13C signals in cis-4,5-dimethylthiazolidine-2-thione) allows an estimation of the puckering in the five-membered monounsaturated heterocycle, thiazolidine-2-thione:[Formula: see text]Application of Lambert's R- value equation provides a value of 41° for the torsional angle between C4 and C5, in contrast to an upper limit value of 25°, deduced from consideration of 13C γ-effects; this disparity is discussed and compared with finite perturbation treatment of 3JHH values.