AbstractSecondary α-deuterium kinetic isotopeeffects confirm that [2+4] cycloaddition between (E)-2-phenylnitroethene and cyclopentadiene occurs in concerted manner, on both the pathway leading to 6-endo-phenyl-5-exo-nitronorbornene and the pathway leading to the corresponding 6-exo-phenyl-5-endo-nitro isomer. According to Hammond terminology the transition states on competitive pathways should be