The effects of humidity, for alternating voltages up to about 700 kV r.m.s. and impulse voltages up to about 1500kV, on gaps between 1m spheres, on rod-gaps, on suspension insulators and on pedestal-post insulators are described. Data are given on cylindrical-post insulators with alternating current and on point-plane gaps with impulse voltages.The data are limited but appear to be sufficient to justify conclusions which could affect practice, in particular, it appears that the a.c. flashover voltage of some gaps and insulators falls at high humidities, so that the negative correction in use at present should be replaced by a positive correction.The data for positive impulses, except for the sphere-gap, show that flashover voltages increase continuously with humidity. The effect with negative impulses is in some cases negligible.