Water penetration into thin sulfonated polyphenylene (sPP) ionomer Films was investigated as a function of time, ionic strength, and film thickness by neutron reflectometry (NR). Understanding the role interfacial effects have on transport across ionic membranes is critical to the design of new responsive thin layers for a variety of applications from fuel cell membranes to protective cloths and water purification. At steady state, a nonuniform distribution of water molecules was observed with a high concentration at the air-polymer interface. An excess of water was also found at the polymer-silicon interface. The mass uptake is initially linear with t(0.5) but crosses over to an anomalous process with extended exposure periods. A delay time for the onset of diffusion is observed and is interpreted in terms of interfacial barrier to diffusion.