Dynamic mechanical thermal analysis (DMTA) on a series of poly(ethersulfone)-amine-cured epoxy resin systems indicates phase separation is occurring, in agreement with previous electron microscopic and dielectric observations. Atomic force microscopy (AFM) measurements reveal that the surface roughness changes with composition and is greatest for the cocontinuous phase structure. Dielectric and gravimetric studies are reported on the absorption of water by these resin systems. Gravimetric measurements yield diffusion coefficients, obtained from the initial slopes, which vary slightly with thermoplastic composition. The equilibrium water uptake decreases with increasing thermoplastic content and when normalized by the volume fraction of epoxy resin exhibits a fairly constant value which infers that the majority of the water is dispersed in the epoxy phase. Analysis of the dielectric data at 10 Hz indicates that behavior similar to that obtained from gravimetric analysis is observed outside the cocontinuous phase region. At the cocontinuous composition, anomalous behavior was observed and is attributed to polarization phenomena associated with the microporous structure of the surface of these polymers. Dielectric analysis indicates that the relative rates of absorption depend on the frequency of observation and that there are two extreme types of environment present in these systems. Some of the water molecules are "bound" to hydroxyl groups generated in the cure process and others exist as "free" molecules in microcavities.