The quantum sieving is a promising approach for the separation of hydrogen isotopes under cryogenic conditions (< 100 K) in microporous solids. To evaluate the role of pore size and guest-cation interaction strength in this process, the adsorption properties of Li-exchanged LTA zeolites (Li 8 Na 4 A and LiA) for D₂ and H₂ were studied. The single gas adsorption isotherms were measured at different temperatures (39-92 K) and the equilibrium D₂/H₂ selectivities were determined using a coadsorption manometric technique. The isosteric heat of adsorption (QD_iso) as a function of loading was determined from the single gas isotherms. It was found that in the partially exchanged material LiD₈NaD₄A the variation of Q iso is smaller than in NaA zeolite. In contrast, in LiA at low loading (< 2 molec/uc) extremely high values were observed (15.0 kJ/mol for D₂ and 13.8 kJ/mol for H 2 ) indicating the presence of strongly adsorbing sites. After the filling of these sites the Q iso decreases and remains almost constant reflecting weaker interactions between the guest molecules and Li + cations located inside the cages (6MR sites). The D₂/H₂ selectivity at 40 K and high loading (~ 11 mmol/g) increases in the order: Li 8 Na 4 A < LiA < NaA. This finding shows that the presence of strongly adsorbing sites in LiA does not allow to increase D₂/H₂ selectivity. The smaller aperture size in NaA than in LiA (due to a larger size of Na + cations) appears thus to be more important factor for quantum sieving in LTA zeolites than the strong guest-cation interactions.