Model composites of spherical glass particles dispersed in a matrix of high density polyethylene (HDPE) were studied both with and without interfacial modification by an azidofunctional trialkoxysilane. The layer coverage was controlled by varying the amount of silane used for modifying the glass sphere surfaces. The dielectric properties of the silane-modified composites were studied as a function of the water content of the samples after equilibrium water uptake. The amount of absorbed water was controlled via the relative humidity of the environment to which the samples were exposed. Measurements were performed as a function of both temperature between 0°C and 30°C and frequency from 100 mHz to 1 MHz. A dielectric interfacial polarization process due to a water layer adsorbed at the glass filler surface was detected. A theoretical interlayer model for the dielectric properties of particulate-filled composites in which an interfacial interlayer is present is shown to predict the detected dielectric loss process. Owing to the interfacial modification by a silane coupling agent, differences in the dielectric properties and water uptake during the absorption process as well as differences in the temperature dependence of the interfacial loss process between the composites can be detected. These changes can be attributed to the effect of the interfacial modification, and this yields insight into the effect of the silane coupling agent on the water adsorption at the filler-matrix interface.