Variational transition state theory has been used to calculate the kinetic isotope effects affecting product ratios in the reaction between 1O2 and d6-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 1O2 with the tritiated derivative of tetramethylethylene.