cis,trans-1,2-Dideuterio-1,4-diphenyl-1,3-butadiene (ct-DPB_d2) was synthesized and its cis–trans photoisomerization in cyclohexane-d₁₂ (C₆D₁₂) at room temperature was monitored by ¹H NMR spectroscopy. The results reveal formation of only trans,trans-1,2-dideuterio-1,4-diphenyl-1,3-butadiene (tt-DPB_d2). The failure to detect formation of trans,cis-1,2-dideuterio-1,4-diphenyl-1,3-butadiene (tc-DPB_d2) eliminates the possibility that an identity bicycle pedal process contributes to inefficiency in the cis–trans photoisomerization of cis,trans-1,4-diphenyl-1,3-butadiene (ct-DPB).