Two methods have been used to detect contamination in evaporated films of silver and germanium by the boats or filaments used as sources. It is shown that a limit of a few parts in ten thousand of tungsten or molybdenum is attainable by microchemical methods. The much lower limit of one or two parts in 107 is attained by the radioactive tracer method. The extent of contamination of films of silver and germanium by tungsten and tantalum boats has been studied using the tracer technique and the minimum contamination attainable found to be of the order of a few parts per million. The possibilities of transfer of the boat material by direct evaporation or by reaction with the residual gas in the system have been minimized; the results suggest that contamination arises by the solution of the boat materials by the molten silver or germanium.