The standard association constants of the ion pair [MgSO4](0), K-ass, in the (0.06455 ethanol + 0.93545 water} were determined from precise e.m.f. measurements of cell (1) without liquid junction: Pt, H-2(101.325 kPa)vertical bar HCl(m(1)), MgSO4(m(2)), (0.06455 ethanol + 0.93545 water)vertical bar AgCl-Ag, (I) where m(i) means molality of species i. The dependence of pK(ass) on temperature T is expressed by the empirical equations: pK(ass) = a(1) + a(2)(K/DT). The other thermodynamic functions of the association process were also calculated and showed that the driving force for the formation of the ion pair is the entropy of association. (c) 2005 Published by Elsevier Ltd.