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Measurements of the x radiation and the electron loading in a niobium cavity resonant in the TM010 mode at 1208 MHz have been made in order to study enhanced electron field emission in superconducting cavities. Measurements have also been made of the low-temperature dc electron field emission from bismuth, niobium, copper, and tungsten utilizing a configuration which approximates the point-to-plane geometry. In the rf cavity measurements, it is found that the x-radiation count rate data follow a modified Fowler-Nordheim equation, which takes into account the time-dependent rf fields, the electron dynamics in the rf fields, the emission of secondary electrons, and the production and absorption of x radiation. Further, it is found that the x radiation from the cavity can be reduced by a factor of 104−106 by helium-ion sputter processing at low temperature. From x-radiation data and electron loading data interpreted with the modified Fowler-Nordheim equation, it has been shown that, after helium-ion sputter processing at low temperature, the electron field emission process can be consistently described by ordinary Fowler-Nordheim electron field emission from geometrical projections at which the electric field is enhanced. Before processing, the electron field emission process cannot be consistently described by ordinary Fowler-Nordheim electron field emission. A point of view which is consistent with the data is that before helium-ion sputter processing the electron field emission is enhanced by resonant tunneling through surface states due to adsorbed atoms, and that as a result of processing, the adsorbed atoms are removed. This point of view is supported by calculated sputtering rates during the processing. In the dc measurements, it is found that the low-temperature electron field emission is consistent with the Fowler-Nordheim theory for an emission tip that has been cleaned by reverse field desorption. However, it is found that the dc electron field emission increases slowly with time, presumably as a result of adsorption of material ejected by electron bombardment of the anode. The resulting changes in the dc electron field emission characteristics are of just the type expected for resonant tunneling through adsorbate surface states, and are remarkably similar to the differences in x-radiation count rate data observed for a superconducting cavity before and after helium-ion sputter processing.