Water at the interface and in the bulk of ultrathin ionomer films confined to platinum surfaces affects the electrochemical activity and proton transport in electrochemical devices, yet it is little explored. This is the first study wherein the water distribution and the hygroexpansion of three different sulfonated fluoropolymers, varying in the composition and length of sulfonic group terminating side chains, as confined thin films (similar to 15 nm) on a Pt surface are examined by neutron reflectometry, ellipsometry, and quartz crystal microbalance. For the shorter side-chain perfluorosulfonated acid (PFSA) ionomers, Nafion EW 1100 and 3M EW 725, one to two monolayers equivalent of interfacial water, similar to adsorbed water on bare Pt, under wet conditions (97% humidity) was observed. In contrast, for the longer chain perfluoroimide acid (PFIA) ionomer, five monolayers equivalent of interfacial water was observed. In the bulk phase of the films, the PFIA ionomer exhibited a multilamellar structure comprising a water-rich layer near the interface region whereas the PFSA ionomer had a more uniform distribution of water. Our study for the first time demonstrates the strong effect of the side chain of sulfonated fluoropolymers on the Pt/ionomer interfacial water.