The behaviour of methanesulfonic and 2,5-dichlorobenzenesulfonic acids in a 20 mass% propylene carbonate (PC)+ ethylene carbonate (EC) mixture has been investigated by means of conductance and potentiometric measurements at 25 degrees C. The conductance behaviour of acids in 20 mass% PC+EC is explained on the basis of simple dissociation (HA reversible arrow H++A(-)) and homoconjugation (HA+A(-)reversible arrow H-2(-)) equilibria. The results of the measurements have been subsequently correlated with potentiometric studies involving the cell : PtH-2(1 atm), HA in 20 mass% PC+ECHgCl2(s)Hg ref. in 20 mass% PC+EC. In addition, the HgHgCl2(s) reference electrode has been used to determine the standard potential of the reaction : quinone+2H(+)+2e(-)reversible arrow hydroquinone vs. SHE in 20 mass% PC+EC. It was found to be 0.3516+/-0.0023 V.