A flow-through potentiometric technique utilizing an yttria-stabilized zirconia (YSZ) pH sensor has been employed to elucidate the effects of NiSO4 and MgSO4 on pH of H2SO4 solutions at temperatures of 200 A degrees C and 250 A degrees C. Solution pH was found to increase with increasing NiSO4 and MgSO4 concentrations at both temperatures. This trend is attributed to the dissociation of NiSO4 and MgSO4 where the SO4 (2-)(aq) released reacts with H+(aq) to form HSO4 (-)(aq). The conversion of measured potentials into pH values was based on the mixed-solvent electrolyte (MSE) model, which is a speciation model of the new OLI Systems(A (R)) software. Both the Henderson equation and exact definition of the diffusion potential were employed in treating the obtained experimental data. Experimental pH values calculated using the diffusion potentials evaluated by either approach are similar, suggesting that the Henderson equation can be effectively used. In addition, LiCl was found to be a suitable alternative to NaCl as the reference electrode solution.