Changes in mechanical stress occurring in unpatterned thin films on substrates at elevated temperatures can yield information about the kinetics of the controlling processes. In this study, a specialized furnace allowed stress at elevated temperatures to be measured by the optical-lever method. The furnace was designed for good temperature control, short thermal transient periods, accurate stress measurement with low scatter, and control of the gas ambient. Stress changes in phosphosilicate glass films measured during exposure to steam at 110–150°C allowed water diffusion coefficients to be calculated. Diffusivity decreased with increasing phosphorus content and with pre-annealing treatments. The effective activation energy for water sorption was about 0.43 eV. Analysis of stress changes associated with crystallization of amorphous silicon at temperatures near 600°C yielded an effective activation energy of 3.1 eV and a density change of about 1% for the transformation.