A series of polyethylene-based nanocomposites was prepared, utilizing silicon nitride or silicon dioxide (silica) nano-powders, and the effect of filler loading and conditioning (i.e. water content) on their morphology and electrical properties was examined. The addition of nano-silicon nitride led to systems that were free of obvious nanoparticle aggregates, whereas the nano-silica based systems showed evidence of aggregation up to the micrometer-scale. While the nano-silicon nitride composites remained essentially dry under ambient conditions, the nano-silica-based composites absorbed appreciable quantities of water from the ambient environment, indicating that interactions with water are dependent on the nanoparticle surface chemistry. Dielectric spectroscopy showed a broad relaxation peak due to adsorbed water at nanoparticle surfaces, which shifted to higher frequencies with increased water content. Similarly, the electrical conductivity was found to be highly sensitive to the presence of absorbed water, particularly for systems containing well dispersed nanoparticles. We conclude that, in nanodielectric applications, nanoparticle surface chemistry is important in determining macroscopic properties, and not just as a means of compatibilizing the filler and the matrix. Additional factors can be critical, here, as exemplified by interactions with water.Dataset to support: Hosier, Ian et al (2016). The effects of water on the dielectric properties of silicon based nanocomposites. IEEE Transactions on Nanotechnology (TNANO).