The forbidden a1 u torsional vibration of C 2F 6 has been observed in far-infrared absorption at 67.5 cm -1 with an integrated intensity of 0.20 ± 0.08 × 10 -2 cm -2. The observed intensity is compared with that expected theoretically from Coriolis coupling of the torsion to the infarred-active a2 u and eu vibrations. For certain force fields and choices of sign for the dipole moment derivatives the agreement is satisfactory. The potential barrier to internal rotation has been recomputed from the observed torsional frequency to be 1367 cm -1. (3.91 kcal mole -1). This value is compared with those of ethane and partially fluorinated ethanes.