The mechanism of the high-temperature, gas-phase isomerization of 1,1,2,2-tetrafluoroethane (FC-134) to 1,1,1,2-tetrafluoroethane (FC-134a) in the presence of H-2 has been explored both experimentally and computationally. Studies of the impact of temperature, H-2/FC-134 ratio, and contact time on conversion and yield, as well as a study of deuterium incorporation when D-2 was used in place of Hz, led to the conclusion that a free radical chain mechanism involving rearrangement of CHF2CF2. to CF3CHF. is involved.