According to Cranberg's hypothesis, tiny aggregates of metallic matter called microparticles, emanating from the electrode surfaces, are responsible for the breakdown of a vacuum gap under high dc stress. Experiments are carried out using a scanning electron microscope to see whether such particles appear within the interelectrode gap prior to breakdown. It was observed that micron‐ and submicron‐sized particles are released in abundance well below the breakdown voltage. Investigations were continued to identify the microparticles with the electrode materials using an electron microprobe analyzer. It was found that the microparticles were composed of the electrode materials. Direct examination of the electrode surfaces, after application of the voltage, under the scanning electron microscope also showed features suggesting that the microparticles are released from the electrode surfaces.