Analysis of residual gases in a typical oil-diffusion pumped and LN2-trapped vacuum system in the 10−7 Torr range under high resolution (greater than 3000) has yielded a number of surprise. It is found, among other things, that the mass 28 peak is a triplet with CO+ dominating both the N2+ and C2H4+ components. Mass 16 is primarily O+ but the remainder of the triplet is primarily NH2+ rather than CH4+ as commonly believed. Mass 44 is primarily CO2+ but two other ions are also present. Alkanes (e.g., C7H15+) and what are believed to be polystyrene fragments are major components near mass 100. Examination of a mercury-pumped vacuum system was made for comparison with the oil-pumped system, and a comparison of the results at several mass peaks is presented.