The amount and size of the charge-balancing cations on the exposed faces of clay particles are supposed to be one of the key factors affecting the specificity of adsorption of gases at clay surfaces. However, the trends characterizing the thermodynamics of gas adsorption, reported for different members of 2:1 phyllosilicate phases, do not systematically follow neither the difference in their layer charge, determining the number of charge-balancing cations, nor the nature of the latter. To better understand the specificity of CH4 and CO2 molecular interactions with different clay phases, the adsorption isotherms were measured for isoionic pure-phase illite and montmorillonite up to pressures of 9 MPa at ambient temperature. For both gases, higher adsorption capacities per unit of specific surface area for montmorillonites in comparison to illites could not be explained by the existing theory relying on the properties of exposed charge-balancing cations on the surface. Instead, the slopes of the isotherms perfectly correlate with the shape of clay particles, characterized by their basal-to-lateral faces’ aspect ratio, which is assessed by derivative isotherm summation (DIS) using argon adsorption at 77 K. Montmorillonite clays, characterized by higher fractions of high-energy hydroxylated particles’ edges (∼45%) in the total specific surface area, featured stronger adsorption interactions with CO2 and CH4 in comparison to illites, for which the contribution of edges to the surface area is only 20%. This introduces a new important factor controlling the mechanism of CH4 and CO2 adsorption by clay minerals.