[en] Highlights: • K–Al-based mixed oxides with mullite structures were synthesized as CO₂ adsorbents. • Al³⁺ sites in the AlO₆ units of the oxides were substituted with various metal ions. • The Fe-introduced sample showed the highest CO₂ adsorption capacity. • Fe substitution induced distortions of the AlO₆ unit, which increased basicity. K–Al-based mixed oxides (KAl₆O_9.5) with mullite structures were synthesized as CO₂ adsorption materials using a polymerized complex method. Al³⁺ sites in the octahedral AlO₆ units of K–Al-based mixed oxides were substituted with various metal ions with +2 or +3 valence states to enhance basicity. Among these samples, the Fe-introduced sample (KAl_5.5Fe_0.5O_9.5) showed 130 times higher CO₂ adsorption capacity than that of Li₄SiO₄. Raman spectra of these samples indicated that large distortions of the AlO₆ unit were observed only for the Fe-introduced sample. Local polarization caused by such distortions could induce increased basicity of this sample.