The pore distribution in activated carbon is crucial for its performance on adsorption processes. In desulfurization applications, the literature highlights the importance of pores from 5 to 10 angstrom. However, little is known and reported about the range of ultramicropores smaller than 4 angstrom, which has been found in commercial samples used for H2S retention. In order to clarify the influence of this fraction of pores on the H2S retention of typical biogas mixtures, measurements of multicomponent adsorption of hydrogen sulfide in the presence of carbon dioxide for activated carbon samples further impregnated with potassium hydroxide have been performed in column dynamics. Molecular simulation, XRF analysis and textural characterization were applied to aid the study. Results have shown that the H2S retention capacity in the presence of CO2 decreased significantly mainly in the impregnated samples. The degree of reduction in H2S retention indicates that ultramicropores can be of substantial importance through a synergistic immobilizing effect of H2S molecules. The better understanding of such an effect could improve the development of more efficient adsorbents for chemical industries. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.