ac breakdown properties of alumina, aluminum nitride, and glass ceramic (SiO2-Al2O3-MgO-TiO2) materials have been investigated. The breakdown strength of glass ceramics (∼65 kV/mm) was found to be higher than that of alumina (∼31 kV/mm) and aluminum nitride (∼16 kV/mm). The specimens were characterized for their density, porosity, and microstructural parameters, such as grain size and distribution, as they are believed to influence the breakdown properties significantly. In all the cases, the prebreakdown and breakdown conduction were found to be associated with visible-light emission at the electrode-specimen interface. The samples that underwent breakdown were found to exhibit irregular puncture channel, terminated with craters at both ends. Several types of crystallization structures were observed at the rim of the craters and in some cases at the edges of the breakdown channels. It is suggested that the breakdown process of the ceramic materials is a combination of electronic, electromechanical, and thermal processes.