The relative activity coefficients of MnSO4 and Mn(ClO4)2 were measured downto about 5×10−5 mol-kg−1 by means of cells with ion-exchange liquid membranes.The measurements at high dilution provide an answer to the age-old problem ofabsolute values of the activity coefficients for MnSO4. They also lead to acorrection of the activity coefficients for Mn(ClO4)2, whose literature values arefound to be biased by ca. 6.5%. Like other bivalent metal sulfates, MnSO4does not obey the Debye—Huckel law inasmuch as negative instead of positivedeviations from the limiting law are observed in the dilute regions. The possibilityfor this kind of behavior to denote ion association is discussed. The best-fitparameters of Pitzer's equation, able to provide accurate values of the activityand osmotic coefficients of the two salts for any concentration, are reported.