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To investigate the hygrothermal ageing of silica/epoxy interfaces in a filled epoxy resin, a macroscopic experimental model of such interfaces has been designed. It consisted of coplanar interdigitated electrodes deposited on a quartz substrate and moulded in epoxy resin. The dielectric properties were followed at 80°C and 80% relative humidity. These properties have been compared with the evolutions under the same conditioning of sheets and mouldings of the unfilled resin. Comparisons show that the resistance of the interdigitated sample was not due to the water-enhanced resin conductivity but to the conductivity of an interphase, which has been quantified. If the interphase is modeled as a uniform water layer at the quartz/resin interface, an estimation of the water thickness in the micrometer can be deduced from the dielectric measurements. This order of magnitude can be useful to understand prebreakdown phenomena resulting from a hygrothermal ageing of composite insulations.