The four chlorinated methanes, methyl chloride (CH3Cl), methylene chloride (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4), were used as model chlorocarbon systems with Cl/H ratios of 0.021 to 0.083 to investigate the thermal stability and hydrodechlorination of chloromethanes in excess hydrogen. The pyrolytic reactions were studied in an isothermal tubular reactor at a total pressure of 1 atm with reaction times of 0.3∼2.0 s at temperatures between 525 and 900 ℃. The thermal stabilities of the chloromethanes, i.e., the temperatures for 99 % destruction within a reaction time of 1 s were 875 ℃ for CH3Cl, 780 ℃ for CH2Cl2, 675 ℃ for CHCl3, and 635 ℃ for CCl4. The less-chlorinated hydrocarbons were more stable, with CH3Cl the most stable chlorocarbon in this reaction system. This work focused on pyrolysis of CH3Cl in an excess-hydrogen reaction atmosphere. The observed hydrodechlorinated products were CH4, C2H4, and C2H6 at temperatures above 850 ℃ in the CH3Cl/H2 reaction system. The number and quantities of intermediate chlorinated products decreased with increasing temperature; the formation of non-chlorinated hydrocarbons increased as the temperature rose. One of main pathways for hydrodechlorinated products resulted from H atom cyclic chain reaction by abstraction. Product distributions along with preliminary activation energies and rate constants are reported. The pyrolytic reaction pathways that describe the important features of reagent decay and intermediate product distributions, based upon thermochemical and kinetic principles, are suggested.