Densification of ZrO2-3 mol.% Y2O3 sol-gel thin films after heating in air at 100 degreesC less than or equal to T less than or equal to 1100 degreesC is investigated, with particular emphasis on porosity evaluation. For this purpose, the refractive index and thickness of the films are determined from transmission spectrometry. Three densification regimes were clearly identified: T < 300 degreesC, 300 degreesC less than or equal to T less than or equal to 800 degreesC and T > 800 degreesC. For porosity evaluation, a simple strategy based only on film thickness measurements is used. These porosity values are compared with those obtained from refractive index measurements, applying commonly used analytical expressions. Our results clearly show that the porosity of thin films is underestimated when the Lorentz-Lorenz expression is used. Implications for preparation of ZrO2 sol-gel thin films are also discussed. (C) 2003 Elsevier B.V. All rights reserved.