Physical adsorption of gases has been revised in terms of the Theory of Micropore Filling as a technique tocharacterise microporous carbons (carbon fiber, carbon molecular sieve, activated carbon and char). Special attention has been paid to the CO₂ adsorption isotherms at 273K, obtained using subatmospheric and high pressures. Becausethe relative pressure range covered by our CO₂ study is similar to the one covered with N₂ at 77K, a suitable comparisonof both adsorptives has been possible. Thus, we have been able to demonstrate that CO₂ is an adsorptive that behavesat 273K similarly to the well-accepted N₂ at 77K. In addition, the results show that the narrow microporous carbonsshould not be characterised only by N2 at 77K, otherwise the results can be useless. We encourage the use of CO₂ (i.e.at 273K) as a complement to N₂ adsorption. This suggestion is especially relevant considering that the most frequentavailable adsorption equipments use only N₂ at 77K and that most of the researchers limit the characterisation ofmicroporous carbons to this adsorptive.