Improved process for vapor phase cracking of dicyclopentadiene and synthesis of 2,3-dihydrodicyclopentadiene therefrom. Initially, a feedstock consisting essentially of dicyclopentadiene and cyclopentene is subjected to thermocracking in the vapor phase, resulting in the conversion of dicyclopentadiene to monomeric cyclopentadiene. The presence of cyclopentene in the feed stock is believed to minimize coke formation on the interior walls of the cracking chamber. Subsequent selective catalytic hydrogenation of the resulting mixture converts substantially all of the monomeric cyclopentadiene to cyclopentene. The cyclopentene, prepared in the above manner, can be combined with additional quantitie;s of dicyclopentadiene and the resultant mixture subjected to dimerization in the liquid phase at elevated temperatures, thereby forming the monomer, 2,3-dihydrodicyclopentadiene.