The far-infrared spectra of 1,3-oxathiolane and its 2,2-d(2) isotopomer have been analyzed in terms of a two-dimensional vibrational potential energy surface expressed as a function of ring-bending and ring-twisting coordinates. The barrier to planarity was determined to be 2289+/-200 cm(-1) while the barrier to pseudorotation, representing the energy difference between the lowest energy twisted form and the highest energy bent form, was found to be 570+/-20 cm(-1). The barrier to planarity is about 500 cm(-1) higher than that of cyclopentane, and this is attributed to the anomeric effect resulting from the -O-CH2-S- linkage which tends to twist the molecule out of the planar configuration.