A density functional theory study at the B3LYP/6-31++G** + RECP(Sb) level of the HF/SbF5 superacid system was carried out. The geometries of possible electrophilic species, such as H2F+.Sb2F11- and H3F2+.Sb2F11-, were calculated and correspond with available experimental results. Calculations of different equilibrium reactions involving HF and SbF5 allowed the relative concentration of the most energetically favorable species present in 1:1 HF/SbF5 solutions to be estimated. These species are H+.Sb2F11-, H2F+.Sb2F11-, H3F2+.Sb2F11-, and H4F3+-Sb2F11-, which correspond to 36.9, 16.8, 36.9, and 9.4%, respectively. Calculations of the acid strength of the electrophilic species were also performed and indicated that, for the same anion, the acid strength increases with the solvation degree. The entropic term also plays a significant role in proton-transfer reactions in superacid systems.