A quartz crystal microbalance was employed to monitor the changes in mass of thin protonated Nafion(R) coatings during the replacement of the protons by other cations. In almost every case the mass of the coatings decreased during the cation exchange, despite the replacement of the protons in the coatings with much more massive cations. The expulsion of water from the coatings, induced by the cation exchange, was responsible for the decreases in mass. With simple aqua cations the quantities of water expelled per cation exchanged were relatively small. However, for multiply charged hexaammine and 2,2’-bipyridine complexes as many as 50 water molecules were shown to be ejected from the coatings for each incorporated cation. Data for the cation exchange of 18 cations are presented and some possible reasons for the accompanying dehydration of the coatings are suggested.