The densification of ZrO2 films prepared by sol-gel synthesis has been investigated by stress measurements, infrared spectroscopy, and spectroscopic ellipsometry after annealing at 150-400 degreesC. It is shown that the tensile stress is limited by viscous flow accompanying the densification, and that the main organic compounds reducing the rate of condensation reactions are acetate groups. The oxidation of these groups at 350 degreesC induces a jump in both the stress and the refractive indices of the films. The effect of film aging in air on the stress has also been studied. The moisture-induced stress change follows a logarithmic time dependence, most likely due to hydration of ZrO2. The sign of stress change is determined by the amount of organic compounds in the gel.