The need for portable biosensing devices has been apparent for several years; however, practical fieldable devices have continued to be challenging to achieve due to their inherent need for sensitivity while maintaining environmental robustness. This is especially true for optical biosensors as small changes in the device's materials' properties or thickness may render the device ineffective. By using Fourier transform infrared spectroscopy and ellipsometry techniques the authors have studied the behavior of silicon dioxide films deposited using electron-gun deposition when exposed to atmosphere and water. Experiments have shown that strain in the film is released when the films absorb water, which can be seen from changes in the Si-O stretching bond and film thickness. (c) 2006 American Vacuum Society.