Performance degradation in membranes for polymer electrolyte fuel cells was discussed theoretically for the case where the membrane is contaminated with foreign impurity cations at the cathode side. Water transport in a two-cation system membrane was considered by assuming an 'infected zone' of finite thickness. Four kinds of boundary value problems were solved, and analytical formulae derived for the water concentration profile across the membrane. The water content in the membrane, the net water flux and the membrane resistance overvoltage were calculated systematically as functions of several relevant parameters in fuel cell operations. Localized contamination at the cathode \ membrane interface turned out to be even more serious than the uniform contamination of the membrane or localized contamination at the anode side. It is noted that special caution should be directed in order to avoid the membrane contamination, especially at the cathode side, because contaminant will easily enter from the air stream through the cathode compartment of a fuel cell.