It has recently been found that particles of contamination reside on surfaces inside vacuum systems as a result of thermal decomposition of glass. These particles contain at least Na, K, and B as well as traces of Al and Si. They may well have been important in many clean surface experiments. Our investigation has shown that these particles have a strong influence in reducing the breakdown voltage in vacuum. Breakdown voltages for Al, Cu, and stainless steel electrodes with radii of curvature of 12.7, 25.4, 50.8, and 101.6 mm were determined under clean and contaminated conditions at 10−5Torr. It was found that under clean conditions the electrodes of smaller radii of curvature have higher breakdown voltages than the more nearly plane electrodes.