The residual gas conditions in numerous commercial oil diffusion pumped coaters have been examined by mass spectroscopy during an extensive program. Particular regard is devoted to organic contamination. The main topics covered are: hydrocarbon partial pressures and their determining parameters in systems equipped with water-cooled and deep-cooled baffles; the influence of oil backstreaming; suitable measures to reduce contamination; a discussion of various contamination sources, for example, the foreline system. The pump fluid was found not to be the most important contaminant in many cases, particularly in coaters working under normal production instead of laboratory conditions. Whereas a slight contamination by pump fluid vapor cannot be avoided completely unless by deep-cooled baffles, all other contaminations are avoidable in principle without such devices. Some operational suggestions are given.