Characterization of microporous solids (activated carbons and carbon molecular sieves) has been carried out by N2 (subatmospheric pressures) and CO2 adsorption (at subatmospheric and high pressures) at 77 and 273 K, respectively. Because the relative fugacity range covered by our CO2 study is similar to the relative pressure range covered with N2, a suitable comparison of both adsorptives can be made. The results of such comparison show that both adsorptives give the same micropore size distribution (MPSD) for open porosity activated carbons. This observation confirms that the adsorption mechanism of both adsorptives is similar. However, carbon molecular sieves, with very narrow microporosity, cannot be characterized by N2 at 77 K, due to the existence of diffusional problems. This is also extensive to many other carbon materials, such as carbon fibers and activated carbons with low degree of activation. As a consequence, in this type of samples, N2 adsorption at 77 K is useless to determine neither the micropore volumes of the narrowest porosity nor their micropore size distributions (MPSD). In this work, the usefulness of CO2 for the characterization of carbon molecular sieves and activated carbons with different activation degrees is demonstrated. In addition, examples of applications that cannot be explained from N2 adsorption but yes by CO2 are presented. As a result, we strongly encourage the use of CO2 (i.e. at 273 K) as a complement to N2 adsorption at 77 K.