In this study, carbonate formation characteristics during solid fuel pyrolysis in CO2 were experimentally studied. Chars derived from coal or rice straw in Ar or CO2 were characterized by Fourier transform infrared spectroscopy (FTIR) and water extraction and electrophoretic procedures. Inorganic element concentrations in chars were measured by inductively coupled-plasma atomic emission spectrometry (ICP-AES). When coal with a K/Si ratio of 0.04 was heated in CO2, carbonate was not formed. In contrast, when rice straw with a K/Si ratio of 2.97 was heated in CO2, carbonate was formed in chars at 973 K under a heating rate of 1 or 30 K/s. The main carbonates formed through the reaction with CO2 were assumed to be potassium carbonates (K2CO3). It was also shown that carbonate in the rice straw chars prepared in CO2 was almost twice of that in the chars prepared in Ar, whereas the total concentration of K in the rice straw chars prepared in CO2 was almost the same as that in the rice straw chars prepared in Ar. Metal oxides or hydroxides in the chars prepared in CO2 decreased significantly. These results suggest that carbonates formed as a result of the reaction of basic oxides or hydroxides with CO2. In fact, the chemical form of potassium species in the chars was altered by CO2 during rice straw pyrolysis.