The Brønsted acidity of synthetic mica-montmorillonite (SMM) clay was studied by periodic DFT calculations. Different structural models were compared to determine the Brønsted acidity of protons of the SMM clay based on (i) isomorphous substitution of Si4+ by Al3+ in the tetrahedral silicate layer and additional NiF-doping (ii) in the platelets and (iii) at the edge terminations of the clay platelets. The acid strength was judged from the computed adsorption energies of ammonia and pyridine. The SMM acidity is mainly determined by the composition of the clay platelets. The strongest acidity is found in structures in which octahedral [AlO]+ is replaced by [NiF]+ adjacent to tetrahedral [Si-(OH)-Al] moieties in the tetrahedral layer. For the Brønsted acid sites in the interlayer of SMM, modification with either Ni2+ or F- in the octahedral layers has only a minor influence on the acidity. Our data indicate that Brønsted acid sites, properly modified in the second coordination shell by electron-withdrawing F, in the interlayer and at defect sites at the edges of clay platelets (intralayer sites) can contribute to the enhanced acidity in NiF-modified SMM. Although the predicted acidity of SMM by ammonia adsorption is higher than that of faujasite zeolite, the reactivity judged from propene protonation demonstrates that zeolites are more reactive than clays. This difference seems to be the result of the curved nature of the micropores of zeolites, which stabilizes the transition states for an acid-catalyzed reaction more than flat surfaces of clays do.