Safety motion-picture film on cellulose acetate base has been found chemically damaged on occasion by storage in the same can with unstable nitrate film. The effects on acetate film of the three nitrogen oxide gases produced by the decomposition of nitrate film have been investigated. Nitrous oxide and nitric oxide were found to be harmless, but nitrogen dioxide, even at low concentrations, is damaging to safety film. The attack of nitrogen dioxide on acetate film is further discussed. Safety films should never be stored in the same can with nitrate films and preferably not in the same room.