The rates of proton transfer in (H3CH..CH3)- and its fully deuteriated derivative are calculated by RRKM theory, including contributions from tunneling for energies below the barrier. These results are compared with rates calculated by canonical transition-state theory, incorporating two different tunneling corrections. For temperatures in excess of ca. 500 K, tunneling may be neglected, with all procedures converging on similar transfer rates. However, tunneling plays a progressively larger role as the temperature is lowered, speeding up the transfer by a factor of 30 at 300 K, and completely dominating the reaction below 200 K. The deuterium isotope effect is quite large ( IO5) at 0 K, remains fairly constant as the temperature is raised to 50 K, and then drops precipitously from 50 to 500 K, after which it levels off to a high temperature limit of 1.12. The curvature exhibited by an Arrhenius plot below 500 K is unambiguously assigned to tunneling. Adjustment of transition-state theory for tunneling by the Wigner correction is incapable of reproducing the above behavior in the temperature regime where tunneling is important. However, an alternate correction, involving a summation of transmission coefficients, provides an improved recipe for incorporating tunneling into transition-state theory.