HCFC-133a(1-chloro-2,2,2-trifluoroethane) was thermally chlorinated in an integral reactor under atmospheric pressure in a temperature range between 573 and 723 K. From the analysis of the data it was confirmed that the reaction was a two-step series-parallel reaction and each step was a second-order reaction. The ratio of the second-step rate constant to the first-step rate constant did not change with temperature and its value was 1.13. The activation energy was 106 kJ/g-mol. These results indicate that the product distribution is the sole function of conversion of HCFC-133a.