Far ultraviolet (FUV) reflectance measurements on in situ deposited MgF2-overcoated aluminum films and on the same I thus after exposure to controlled doses of air show that a reflectance degradation up to 10% takes place due to exposure. We applied increasing doses of molecular nitrogen, molecular oxygen and water vapor in order to identify the components of the atmosphere which are responsible for the variation mentioned above. Results show that doses larger than 10(4) L of both molecular oxygen and water vapor produce a reflectance decrease. On the other hand molecular nitrogen doses up to 10(6) L do not modify reflectance at all. Additional FUV transmittance measurements on MgF2 films deposited onto crystalline MgF2 substrates also showed a degradation behavior under similar gas exposures. (c) 2005 Elsevier B.V. All rights reserved.