Abstract Potential changes of cells with a glass electrode and a hydrogen electrode in combination with a reference half-cell were measured in solutions of various compositions of cations at constant concentrations of hydrogen and total perchlorate ions. Measurements were carried out for the systems LiClO4-NaClO4, LiClO4-AgClO4, NaClO4-AgClO4, NaClO4-TlClO4, LiClO4-Mg(ClO4)2, LiClO4-Ba(ClO4)2, NaClO4-Ba(ClO4)2, AgClO4-Ba-(ClO4)2, and LiClO4-La(ClO4)3. In some cases a remarkable potential change was observed. Part of the change was due to the variation of the liquid junction potential at the interface of a test solution and a solution in a salt bridge. However, the change was attributed in greater part to the electrostatic free energy change of the transfer of hydrogen ion from a (3/m m Am+, 3m ClO4−) solution to a (xm Am+, ym Bn+, 3m ClO4−) solution (where mx+ny=3). Variation of the composition of electrolytes gives rise to a significant change of the dielectric constant of a solution, so that the activity coefficient of hydrogen ions varies in different solutions at a constant concentration of perchlorate ions. Potential changes are additive in A1(ClO4)m1-B1(ClO4)n1, A1(ClO4)m1-B2(ClO4)n2 and B2(ClO4)n2-B1(ClO4)n1 solutions for a given replacement of cations in solutions, where A1 B1 and B2 denote cations with charges of m1 n1 and n2, respectively.