[en] The gas phase thermal decomposition of various isotopically substituted 1,4-cyclohexadienes was studied in the 275-375⁰C temperature region to obtain transition state information for the Woodward-Hoffman symmetry allowed hydrogen elimination reaction. Competitive decompositions with (i) a 1:1 mix of 1,4-cyclohexadiene and perdeuterio-1,4-cyclohexadiene or (ii) 1,2,3,4,5,6-hexadeuterio-1,4-cyclohexadiene produced (1) H₂ and D₂ or (2) H₂, D₂, and HD. The difference in E/sub a/ for H₂ and D₂ elimination was 2.1 +- 0.4 and 2.4 +- 0.8 kcal/mol for systems (i) and (ii), respectively; the difference in E/sub a/ for H₂ and HD was 1.5 +- 0.4 kcal/mol. The results indicate a tight transition state which resembles benzene in its vibrational frequency assignment while the reaction coordinate is predominately the concerted breaking of two carbon-hydrogen bonds