Nitrogen adsorption isotherms on activated carbon fibers (ACFs) were measured at 77.3 K in the relative pressure range from 10(-7) to 0.995. The low relative pressure data are examined in detail in order to characterize the microporosity and the surface heterogeneity of the samples studied. The pore structure of ACFs was studied by using the ct, plot and DFT methods. Adsorption potential and adsorption energy distributions were also calculated. The results showed that ACFs are highly microporous and in some cases contain small mesopores (2-4 nm). ACFs exhibit a significant structural and energetic heterogeneity.