SBA-15 mesoporous silica was functionalized with polyethylenimine and was used as a substrate for CO2 adsorption. The synthesized material, denoted SBA-15-PEL was characterized by means of X-ray diffraction, thermogravimetric analysis, and N-2 adsorption/desorption at 77 K, in order to prove that polymer chains efficiently filled the pores of functionalized samples. CO2 adsorption isotherms on SBA-15-PEI were evaluated at T = (298, 313, 328, and 348) K for pressures up to 100kPa by means of a volumetric technique. The experimental data showed a significant dependence of the CO2 adsorption capacity on temperature, with the highest capacity encountered at the highest temperature explored. Despite this unusual behavior, CO2 adsorption on SBA-15-PEI was satisfactorily modeled by means of the Sips isotherm. The modeling effort allowed to evaluate the isosteric heat of adsorption as a function of the fractional coverage of SBA-15-PEI. The comparison between the results obtained in the present work and those relative to CO, adsorption on "benchmark" microporous substrates, such as 13X zeolite and Cu-BTC metal organic framework, allowed us to highlight significant analogies and differences with those other solids, giving interesting hints on the possible applications of SBA-15-PEI.