Measurements of the electric breakdown fields of films of NaCl, LiF and cryolite (Na₃AlF₆) are reported. The films were tested in vacuum, immediately after deposition, with voltages increasing linearly at a rise rate which was normally about 10 V msec^-1. The electrodes were of evaporated Ag, Al, or Au. Ag appears to behave anomalously, possibly as a result of migration in the strong applied field.The results are lower than would be expected on the basis of published values for bulk single crystals, the breakdown field in a film 1000 Å thick being of the order of 1 MV cm^-1 in all the dielectrics studied. The breakdown field was found to increase with decreasing film thickness, and also, in cryolite, to increase with the rise rate of the applied voltage. Some evidence that the latter effect also occurs in NaCl and LiF was found.It is suggested that the depression of the breakdown field is due to the structure of the layers. Thus it may not be correct to assume that a disordered structure always leads to an increased electric strength. In view of the possible effects of the structure, the thickness dependence of the breakdown field cannot be taken as conclusive evidence for an ionization mechanism of breakdown.