A thin (1-2 mil) Be foil is generally used as a window in a Si(Li) detector system. It was replaced by an aluminized Mylar foil (0.5 mg/cm2 thick aluminium of 99.999% purity coated on a 1 mil thick Mylar foil). The aluminized Mylar window is found to be a useful and simple substitute for the Be window. © 1979.