Cycloaddition of tetracyanoethylene to 2-methyl-1′,1′,1′-d 3 -7-methyl-2,trans-4,6-octatriene-1,1,1-d 3 (DMOT-d 6 ) shows a small inverse steric ß-secondary isotope effect for the [4 + 2] path (k H /k D =0.95±0.05), which is consonant with a concerted mechanism. On the other hand a substantial remote η-secondary isotope effect (k H /k D =0.81±0.05) was measured for the [2 + 2] path. This isotope effect, is the most remote reported in the literature, and it is a result of hyperconjugation in the dipolar intermediate which is formed in the rate determining step for the [2 + 2] path.