A radioactive tracer technique is proposed as a quick, simple, nondestructive method for measuring the deposition of diffusion pump oil and as a possible means of evaluating the efficiency of vacuum components. A simple bell jar vacuum system was assembled consisting of a 4-in. oil diffusion pump, a multicoolant baffle, a 4-in. gate valve, and a bell jar with associated gauges. Ten milliliters of Dow Corning 704 oil were irradiated in the low intensity test reactor at ORNL for 2.6 h. The 30Si14, comprising 0.54% by weight of the oil, becomes activated to 31Si14. The system utilizing various baffle coolants, was operated for certain time intervals. The results indicate the presence of an initial backstreaming surge and also show the relative effectiveness of air-, water-, and liquid-nitrogen-cooled baffles. Deposition rates of a few Å∕h were measured.