It is known, that by addition of zirconium layered phosphate (ZP) to perfluorinated sulfonic acid ionomers (PFSA), the proton conductivity of the composite decreases, but the high temperature stability and electrical efficiency increases as compared to the unmodified PFSA membrane. In order to clarify this finding, mechanical properties and conductivity of a commercially available PFSA ionomer (Nafion®117), PFSA-ZP and PFSA-calcium phosphate (CP) composites were examined ex-situ and in-situ under conditions similar to fuel cell application. The results indicate a strong columbic interaction between the organic and inorganic ZP phase, which at the same time increases the stiffness and Tg by ionic cross-linking and decreases conductivity. The interaction is more pronounced at low water activity. A threshold of about three water molecules per sulfonic acid group was identified, above which a significant increase in proton conductivity is achieved and, concomitantly, most of the mechanical stiffening effect has vanished. CP as inorganic additive, however, has a tendency to be washed out of the ionomer, most probably due to the high acidity of Nafion®.