The combination of a low-loss polymer film with a low-loss impregnating oil does not always give a low-loss composite insulation, particularly when the system is aged under simultaneous thermal and voltage stress. This anomalous behavior appears to be related to a loss phenomenon at the oil-to-polymer interface. For the combination of fluorinated ethylene propylene (FEP) film and polybutene oil a remarkable reduction in dissipation factor was achieved by increasing the degree of wetting at the interface. This was brought about by modifications tending to increase the surface energy of the solid (polymer) and to decrease the surface tension of the liquid (oil). The resulting system gave low and stable dissipation factors on extended aging and, in addition, did not show loss peaks as a function of voltage as observed on the original insulation.