Electrical conductivities of very dilute aqueous solutions of magnesium sulfate and nickel sulfate (similar to 10(-3) mol.kg(-1)) were measured at temperatures from (398.15 to 548.15) K and from (398.15 to 498.15) K, respectively, with an alternating current (AC) conductivity flow cell at a pressure of 18.62 MPa. The resulting conductivity data were modeled using the Fuoss-Hsia-Fernandez-Prini (FHFP) conductance equations together with the appropriate activity coefficient expressions, to obtain equilibrium constants for the ion association reactions: Mg(2+)(aq) + SO(4)(2-) (aq) reversible arrow MgS(4)degrees(aq) and Ni(2+)(aq) SO(4)(2-) (aq) reversible arrow NiSO(4)degrees(aq). The equilibrium constants K(A) for the two systems, which correspond to the formation of all types of ion pairs (solvent-solvent-separated, solvent-separated, and contact ion pairs), agree remarkably well with results calculated from solubility measurements at much higher concentrations. The results were described by the temperature-dependent equilibrium constant model to a precision of +/- 0.03 in log K(A).