Usually, nitrogen and argon adsorption�desorption isotherms are used at their respective boiling points for the determination of specific surface area via the BET theory of microporous materials. However, for ultra-micropores, where nitrogen and argon cannot access at cryogenic temperatures, the CO2 adsorption�desorption isotherms have been considered as alternative options for the determination of specific surface area by extending BET theory, but the surface area determined by using CO2 adsorption�desorption isotherms is not significant due to strong CO2-CO2 interactions. In this study, the microporous covalent organic polymers are subjected to nitrogen and CO2 adsorption�desorption isotherms and the results showed that a clear linear region is available in isotherms, which confirms the presence of ultra-micropores. The surface area determined by the CO2 adsorption�desorption isotherms is higher than the surface area determined by N2 adsorption�desorption isotherms. These results indicate that the microporous covalent organic polymers contain ultra-micropores where only CO2 can reach, while nitrogen and argon cannot access at cryogenic conditions because their kinetic diameter is larger than CO2. © 2020, Springer Nature Switzerland AG.