Calorimetrically determined log K, ΔH°, and ΔS° values valid in aqueous solution at 25° and zero ionic strength are reported for the interaction of SO42– with Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Al3+, Ga3+, ln3+, Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+. The results are compared with predictions of electrostatic theory and with the results of reaction rate studies.