Electrical breakdown in single crystals of NaCl, NaCl+MnCl2, KCl, and KCl+SrCl2is studied in the temperature range 100°–320°C. Two types of breakdown are distinguished and detailed study is given to that type in which no strong dependence of the breakdown strength on the time of application of voltage is found. ``Intrinsic''‐type mechanisms would appear to be involved, but the nature of breakdown and of the prebreakdown current is such as to suggest that space‐charge effects, if present, are not primarily of ionic origin as has been previously suggested.