Conditions for the deposition of MgF2(s) from molecular beam source of MgF2(s) under a residual atmosphere including water vapor H2O(g) are calculated using thermodynamic data of the basic quaternary H-F-Mg-O system, Calculations show that the residual water vapor is gettered on the MgF2(s) source during its heating to form MgO(s), without affecting significantly the composition of the deposited films, The interaction of water vapor with the deposited MgF2(s) film is negligible for substrates maintained at room temperature meanwhile for a substrate temperature equal to 600 K, the MgO(s) formation on the deposited film becomes significant, the ratio MgO(s)/MgF2(s) being equal to 1%, a value corresponding approximately to the oxygen content measured experimentally in the deposited films,