The adsorption of CO2 up to pressures of 4 MPa has been studied using two series of activated carbon fibers (ACFs) covering a wide range of burn-off. The relative fugacities covered in these experiments range from 3 x 10(-4) to 0.76. Additionally, N-2 adsorption at 77 K and CO2 adsorption at 273 and 298 K at subatmospheric pressures have been carried out. The experiments performed at high pressures allow us to compare both adsorptives at similar ranges of adsorption potential. The results obtained led to the following conclusions : (i) CO2 adsorption at 273 K at subatmospheric pressures is a suitable technique to characterize the narrow microporosity of the ACF. (ii) The use of N-2 to characterize the narrow microporosity is not appropriate because its adsorption is limited by the existence of diffusional restrictions in this type of porosity. (iii) CO2 at 273 K (or 298 K) is an adsorptive that behaves quite similarly to Nz at 77 K at comparable relative pressure ranges; thus, CO2 adsorbs in the super-microporosity range (pore size : 0.7-2 nm) at 298 K if pressures of about 4 MPa are used.