It is widely proposed that the global warming is mainly caused by the increase in of carbon dioxide (CO2) content in the atmosphere, and that fossil fuels are the dominant sources of this CO2. Therefore, the quality of fossil fuels tends to be evaluated by amounts of CO2 emissions. For the evaluation of an oil shale from this point an on-line thermogravimetric-gas chromatographic system was used to measure CO2 evolution profiles on temperature with a small oil shale sample. This method makes possible to estimate the amounts of CO2 evolved from kerogen and carbonates in retorting and those from carbonates in combustion, respectively. These results will be basic data for a novel oil shale retorting process for the control of CO2 emissions. The profiles for Thai and Colorado oil shales have shown CO2 mainly evolved by the pyrolysis of kerogen below 550 degrees C, and that evolved by the decomposition of carbonates above that temperature. On the other hand, the profile for Condor oil shale showed that most carbonates decomposed below 550 degrees C, while only small amounts of carbonates decomposed above this temperature.