Nuclear magnetic resonance and certain conductivity data are reported for solutions of perchloric and fluorosulfuric acids in sulfolane. Additional measurements were made on solutions of disulfuric and hexachloroantimony (V) acids. HSbCl6is a strong acid in this weakly basic dipolar aprotic solvent. HClO4, HSO3F, and H2S2O7are incompletely dissociated withK = 10−2.7, 10−3.3, and ~ 10−5, respectively. The acid strength in dilute solution in sulfolane follows the order HClO4 > HSO3F > H2S2O7, which differs from that known in H2SO4which is a strongly associated polar protic solvent.