The continuous-wave breakdown of air and an air/carbon-tetrachloride mixture in uniform electric fields is examined, at frequencies between 3.5 and 30 Mc/s.The observations in air indicate that, if the pressure is reduced below atmospheric, the high-frequency breakdown voltage does not fall below the 50 c/s value until the amplitude of positive-ion oscillation has become much smaller than the gap width. This result is explained by a breakdown mechanism which depends both on positive-ion accumulation and on the variation of electron-ionization coefficient with electric field strength. Measurements by an air-stream method of a quantity related to pre-breakdown ionization in the gap appear to be consistent with the suggested breakdown mechanism. The h.f. measurements also exhibit the electron-diffusion type of breakdown, where a simple theoretical relationship must be satisfied for discharge initiation.A description is given of the effect of a small admixture of carbon tetrachloride vapour on the h.f. breakdown of air at reduced pressure. The vapour concentration was insufficient to affect 50 c/s breakdown. Where the electron-diffusion type of breakdown is normally operative, the h.f. breakdown strength increases by up to 50% when the vapour is present. The increase appears to be caused by electron attachment to vapour molecules.