Do 1,6-dioxa-6a lambda (4)-thiapentalene (1) and 1,6,6a lambda (4)-trithiapentalene (2) have C-s symmetry, with a double-well Potential, Or C-2v symmetry, with a single-well potential? The long-standing question of their structure in solution is answered unequivocally using the NMR method of isotopic perturbation. Statistical mixtures containing zero, one, and two deuteriums were synthesized and observed by C-13 NMR spectroscopy. The isotope shifts of C5 are +15 in 1-alpha -d and -70 ppb in 2-alpha -d, which are small and correspond simply to an intrinsic isotope shift ((5)Delta (0)). Since there is no large downfield equilibrium isotope shift, both I and 2 are symmetric, with the motion of the central sulfur described by a single-well potential. The symmetry of 2 is not reduced by added mercury(II) chloride.