Measurements of the reflection coefficient of HCl and CaCl2 of a phenolsulfonic acid-formaldehyde cation exchange membrane in contact with mixed aqueous electrolyte solutions containing HCl and CaCl2 are reported. In the system HCl(Delta c)/CaCl2, the reflection coefficient of HCl is found to be positive starting from sigma(HC) approximate to 0.8 and to reach maximum values of about sigma(HCl)approximate to +2.1. In the system CaCl2(Delta c)/HCl, the reflection coefficient of CaCl2, starting from sigma(CaCl2)approximate to +0.4 becomes negative with minimum values of about sigma(CaCl2)approximate to -0.7. The composition dependence of sigma(HCl) and that of sigma(CaCl2) are discussed in terms of the model of the membrane with narrow pores.