The impulse breakdown of sulphur hexafluoride and nitrogen gas has been investigated at pressures up to 1.5MN/m2 and 5MN/m2 (15 and 50atm), respectively, for voltages up to 700kV, in order to determine the effect of covering the electrodes with insulating film, under both uniform-and coaxial-field conditions. The first breakdown fields are given, as well as the conditioned values, as these are of practical importance. In general, insulated coverings markedly increased the breakdown field; with uniform-field conditions, they enabled Paschen's law to be obeyed for fields up to 130 MV/m in sulphur hexafluoride, and 40 MV/m in nitrogen, about 50% higher than the fields for which it is normally valid with uncovered electrodes at these pressures. Dust particles were shown to have a deleterious effect, particularly at the higher pressures and fields, and could lower the breakdown field by 30%. Polarity effects on breakdown were found for both gases, with covered and uncovered electrodes.