Micropore size distributions of various carbonaceous materials were obtained using two methods based on the Dubinin Radushkevich equation and the relationship between the characteristic parameter of this equation and the pore width. One method was the well-known Dubinin Radushkevich Stoeckli (DRS) equation, and the other method used the integral adsorption equation without assuming a mathematical distribution for dw(0)/dE(0). These methods were applied to carbon dioxide adsorption data on four activated carbons and eight chars to obtain micropore size distributions. Significant differences were found between the results obtained by the two methods. The DRS method gives an average of the distribution obtained by the method using the integral adsorption equation, because the latter is not constrained to a particular preassumed shape. The results reveal that the differences are more significant for the analysis of adsorption data on chars than on activated carbons. This was attributed to the fact that a less-homogeneous pore structure is expected to exist on chars than on activated carbons that cannot be described by the DRS method and can be described by the method based on the integral adsorption equation.