Considerable increase in compressive stress was observed in silicon dioxide films upon prolonged exposure to atmospheric moisture. The compressive stress change upon exposure to atmosphere consisted of a reversible change and an irreversible change. The irreversible component was found to be due to structural changes in the Si-O-Si network. The reversible change in stress is due to water molecules absorbed on walls of the pores. (C) 1999 American Institute of Physics. [S0003-6951(99)01348-0].