We investigated the thermal stability of exo-tetrahydrodicyclopentadiene (exo-THDCP, C10H16) in the absence and presence of 1,2,3,4-tetrahydroquinoline (THQ), which acts as a hydrogen donor (H-donor). It was found that conversion of exo-THDCP was faster at the higher temperature. The increase in the rate of = C-10. Conversion of exo-THDCP was found to be slowed in the presence of THQ The THQ-induced decrease in the conversion of exo-THDCP was smaller at the higher temperature. In addition, the THQ-induced decrease in the rate of = C-10. We proposed the mechanism for the thermal decomposition of exo-THDCP, which explains the THQ:induced thermal stability improvement of exo-THDCP and suggests a mechanistic basis for developing effective additives capable of lowering C-n formation at high temperatures.