Variational transition state theory has been used to calculate the kinetic isotope effects affecting product ratios in the reaction between O-1(2) and d(6)-tetramethylethylene. The minimum energy path on the potential energy surface for this process reaches a valley-ridge inflection point and then bifurcates leading to the two final products. Using canonical variational transition state theory, two distinct dynamical bottlenecks were located corresponding to the H- and the D-abstraction, respectively. The calculated KIE at 263 K turns out to be 1.126. Analogously, a H/T KIE of 1.17 at the same temperature has been found for the reaction of O-1(2) with the tritiated derivative of tetramethylethylene.