The efficiency and face seal leakage characteristics of two half masks equipped with particle filters or gas filters, and of two surgical masks were studied by means of a test head connected to a breathing machine. Filtration and leakage were studied as a function of particle size over a diameter range of 0.3-10 micron with corn oil aerosol and an optical particle counter. The filtration efficiency of the filter materials was good, over 95%, for particles above 5 micron in diameter but great variation existed for smaller particles. The face seal leakage was manifested as decreased efficiency for large particles and also for total mass, while the particles in the micrometer range contained the major part of the test aerosol mass. The particle number efficiency diagrams obtained can be used both in filter material studies and in leak detection of valves or filter housings.