Composites of epoxy resins filled with hollow microspheres of ceramics, glass and polymer were studied in an attempt to produce a material of low dielectric constant. The accumulation of interfacial water was tested for exposures to 85% humidity at 85-degrees-C for periods of up to 1 000 h. Water absorption was monitored by gravimetry and dielectric measurements. With all but one type of microsphere, a dramatic increase of the dielectric constant developed after durations of exposure which were of the order of the Fickian diffusion time of samples of unfilled resin. Unfilled samples, on the other hand, did not exhibit this behaviour. Impedance spectroscopy carried out at frequencies of up to 1 GHz gave evidence for two distinct modes of ionic conduction : about the spheres and across the bulk of the sample. Microspheres of acrylonitrile-vinylidene chloride copolymer in amine-cured epoxy resins did, however, remain stable and therefore yielded a reliably low dielectric constant.