An experimental research program has demonstrated that the high‐voltage performance of a vacuum gap can be improved by the simple expedient of coating the cathode surface with a suitable thin insulating film. Steady voltages ranging to 340 kV were obtained with 5‐mm gaps composed of 15‐cm‐diam Rogowski electrodes. Simultaneously the average gap currents were suppressed to the 10−8to 10−10A range. Further, it was shown that an insulating film on the anode can be severely detrimental to gap performance. Experiments were performed, at voltages up to 380 kV, involving 12 different film materials. Film thicknesses ranged from 0.2 to 135 &mgr; and film resistivities ranged from 1011to 1019&OHgr;‐cm. The dielectric constants were between 1.7 and 3.5, except for one at 90. Based upon the experimental results and their interpretation, a tentative set of specifications can be given for a good cathode film: (1) resistivity of at least 1011&OHgr;‐cm; (2) dielectric constant in the range 1.5 to 4; (3) dielectric strength of at least 106V/cm; (4) film thickness between 10 and 25 &mgr;; (5) mechanically hard and smooth with high abrasion resistance and high adhesion strength; (6) no gas bubbles within the film. If bubbles are unavoidable they must be substantially smaller than the film thickness; (7) low vapor pressure; (8) the cathode substrate should be of a highly polished metal suitable for quality deposition of the desired film material.