The standard potential of the (Ag, AgCl) electrode E-o, the standard second ionization constant of sulfuric acid K-2, and the standard association constant of the ion pair [MgSO4](o), K in (0.03226 CO(NH2)(2) + 0.96774 H2O) at five temperatures from 278.15 K to 318.15 K were determined from precise e.m.f. measurements of different cells without liquid junction. The dependence of pK on temperature T is expressed by the empirical equation pK = 6.617 - 777.9 . (K/T) - 0.02001 . (T/K). The other thermodynamic functions of the association process were also calculated and show that the driving force for the process is the entropy of association.