This study shows that for a reliable evaluation of porous adsorbents for carbon capture based on the fixed bed adsorption analysis, one must consider the effect of velocity variation due to adsorption to make a fair judgment on predicting the performance of materials under flow conditions. A combined experimental and numerical study of CO2/N-2 adsorption in fixed beds using three forms of adsorbents of amorphous powder (bulk activated carbon, AC), crystalline powder (bulk CuBTC metal-organic framework, MOF) and crystalline pellets (pelleted CuBTC) was carried out to show the effect of velocity variation on CO2 breakthrough curves. Significant deviations are observed in the estimated amount adsorbed calculated from fixed bed experiments when models used for interpretation the measured data consider constant gas velocity because the stoichiometric time is underestimated. We show that the difference in breakthrough times estimated in models that consider constant and variable gas velocity grows exponentially with the feed gas concentration. (c) 2018 American Institute of Chemical Engineers AIChE J, 64: 2189-2197, 2018