5,6-Dihydro-2H-thiopyran, CH2CH2CH=CHCH2S, has been synthesized and its far-infrared and Raman spectra recorded. Two series of sharp bands were observed originating from 139 and 235 cm-1 in the infrared spectrum for the out-of-plane ring-bending and the ring-twisting vibrations, respectively. A detailed energy level diagram including numerous excited states was determined for the two coupled vibrations. The two-dimensional potential energy surface, which satisfactorily fits more than two dozen observed transitions, was calculated to be V = 2.431 × 104x41 - 0.383 × 104x 12 + 2.258 × 104x24 - 1.966 × 104x22 + 1.026 × 10 5x12x22, where x 1 is the ring-bending coordinate and x2 is the ring-twisting coordinate. The minimum energy on the potential surface corresponds to a twisting angle of 37.8° (the half-chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 4130 cm-1 above the twisted conformation on the potential energy surface. The results are compared to analogous molecules and to molecular mechanics calculations.