Thermal reaction studies of diluted mixture (1%) of chloroform (CHCl3) under each argon (Ar) and hydrogen (H2) reaction atmosphere have been investigated to examine the effect of reaction atmosphere on decomposition of CHCl3 and product distributions. The experimental results were obtained over the temperature range 525-900 ℃ with reaction times of 0.3-2.0 sec. at 1 atm by utilizing an isothermal tubular flow reactor. Complete destruction (>99%) of the parent reagent, CHCl3 was observed near 675 ℃ under H2 reaction atmosphere (CHCl3/H2 reaction system) and 700 ℃ under Ar reaction atmosphere (CHCl3/Ar reaction system) with 1 sec reaction time. The CHCl3 pyrolysis yielded more conversion in H2 atmosphere than in Ar atmosphere. Major products in CHCl3/Ar reaction system were C2Cl4, CCl4, C2HCl3 and HCl over a wide temperature range. Hydrocarbon was not found in CHCl3/Ar reaction system. Major products of CHCl3/H2 reaction system observed were CH2Cl2, CH3Cl, CH4, C2Cl4, C2HCl3, C2H2Cl2, C2H3Cl and HCl at 600 ℃ with 1 sec. reaction time. Non-chlorinated hydrocarbons such as CH4, C2H4 and C2H6 were the major products at above 850 ℃. Product distributions were distinctly different in Ar and H2 reaction atmospheres. The H2 gas plays a key role in acceleration of reagent decay and formation of non-chlorinated light hydrocarbons through hydrodechlorination process. The important reaction pathways, based on thermochemical and kinetic principles, to describe the features of reagent decay and intermediate formation under each Ar and H2 reducing reaction atmosphere were investigated.