Five possible mechanisms of dc vacuum breakdown are analyzed for rf electric fields. The theories which are considered are the vaporization of a micron‐sized cathode irregularity due to joule heating by field‐emitted electrons, vaporization of the anode due to bombardment by field‐emitted electrons, field‐emission current instability created by the presence of anode ions, exchange of electrode ions, and microparticle vaporization. The analysis indicates that mechanisms which do not involve anode ions in gaps of 0.2–1.0 cm will not be strongly influenced by rf fields in the frequency range of 20–120 MHz. Under similar conditions, the theories involving anode ions predict a very sizable increase in the voltage capability of vacuum‐insulated electrodes. Experimental ratios of vacuum breakdown voltage at 21.5 MHz to that for 60 Hz were experimentally determined for highly polished tungsten electrodes with separations between 0.18 and 0.61 cm. The rf voltages obtained experimentally were less than the values predicted by theory, although the cathode whisker vaporization and ion perturbation by light ions were within experimental uncertainties. From the differences seen in spark conditioning of electrodes, it would appear that micron‐sized particles are involved in the spark threshold at 60 Hz.