The Horvath-Kawazoe model is adapted to calculate the ultramicropore size distribution of carbogenic materials from the methyl chloride adsorption isotherm at 273 or 295 K. For Takeda 5A carbon, it is shown that the pore size distribution and pore mode of 0.5 nm obtained from methyl chloride adsorption at 273 K are similar to those obtained by Horvath and Kawazoe from nitrogen adsorption at 77 K and probe molecule adsorption studies. The pore size distribution (pore mode = 0.56 nm) also was obtained for Carbosieve G, and the result is shown to be independent of the temperature at which the isotherm for methyl chloride was obtained. For poly(furfuryl alcohol)-derived carbogenic molecular sieve, the pore size distribution is similar to that of Takeda 5A with pore mode at 0.5 nm. Using this method, even though Ambersorb 1500 and Carbosieve G display similar micropore volumes and BET surface areas, they exhibit different pore size distributions.