The flashover characteristics of pressboard, polycarbonate, polyphenylene oxide, Permali and Perspex have been measured under ramp function pseudoimpulse voltage application conditions in clean, dry, degassed liquids with permittivities varying over the range εr = 2–5. The solid materials were tested in the form of 5×10−3 m-thick discs, 12×10−3 m in diameter, between plane parallel brass electrodes. The effect of introducing solid particulate contamination into the liquid and that of covering the electrodes with very thin insulating coverings have also been measured. The results obtained in this work indicate that the flashover mechanism is initiated in the liquid immediately adjacent to the spacer/liquid interface as a consequence of electric-field intensification induced by permittivity mismatching between the solid and the liquid. Furthermore, under conditions where the liquid is clean, the flashover voltages follow an extreme distribution when the permittivity of the liquid is less than that of the solid and a normal distribution when they are equal or when the liquid has the greater permittivity. As a result of this, the 1% probability breakdown value of the interface increases significantly when the permittivities of the solid and liquid are matched, whereas the mean breakdown value is hardly affected.