Ten types of particles commonly found in electron tubes were tested for their effects on high-voltage breakdown and interelectrode current. The particles were applied to stainless steel electrodes and then tested for initial breakdown voltage, number of breakdowns necessary to condition the electrodes, and interelectrode current after high-voltage conditioning. It was found that insulating particles lower the breakdown voltage as much as 30% but the electrodes can be easily conditioned with little damage. Conducting particles and amorphous carbon particles lower the breakdown voltage up to 45%. In addition, these particles cause severe high-voltage conditioning problems. There is much more arcing during conditioning which results in severely damaged electrode surfaces. The arc craters are then a source of interelectrode leakage.