Ab initio quantum-chemical calculations reveal that the interaction of isobutene with H3SO4+ ions produced by self-dissociation of sulfuric acid occurs virtually without an activation barrier, whereas the reactions involving neutral species of sulfuric acid are characterized by considerable activation barriers. 14.9 kcal/mol at the MP2(full)/6-31+G*//6-31+G* level and 16.9 kcal/mol(-1) at the MP2(full)/6-31+G*//3-21 + G* level. It is also concluded that the species resulting from interaction of isobutane with H3SO4+ ions of protonated tert-butyl sulfuric acid are ion-molecular complexes which should be considered as tert-butyl carbenium ions weakly solvated by H2SO4 molecules. Although the concentration of these species in concentrated sulfuric acid is very low, presumably, they play a role of active intermediates in isobutene conversions catalyzed by sulfuric acid.