A conductivity cell, developed previously, was employed to investigate the speciation of MgSO4-H2SO4 solutions particularly at high temperatures. These solutions were measured from 15 to 250 degrees C at the equilibrium vapor pressures. The maximum measured concentrations of MgSO4 and H2SO4 were 0.30 m and 0.45 m, respectively. At 250 degrees C and constant H2SO4 concentration, the solution conductivity drops with the increase of MgSO4 concentration. It is postulated that this drop is caused by a decrease in the concentration of H+ due to bisulfate formation. However, the limiting equivalent conductivity of H+ was not significantly affected by MgSO4. Application of the extended mean spherical approximation (MSA) theory to the experimental data allowed the coefficients that determine the ionic strength dependence of the average effective radius to be obtained. With these coefficients, the conductivity of the MgSO4- H2SO4 aqueous system can be reproduced up to 250 degrees C with an average absolute difference of 2.47%.