In situ spectroscopic ellipsometry measurements in the ultraviolet-visible spectral range were performed during metalorganic chemical vapor deposition of GaN and AlGaN layers on sapphire. At first. the initial GaN-sapphire interface formation has been characterized and the optical response of the nitride layers for temperatures up to 1200 degreesC has been recorded. The data were used as the basis for evaluating kinetic ellipsometry measurements performed during growth: an algorithm, based on the virtual interface approximation model, has been developed and implemented to determine the layer composition of AlGaN multilayer structures in real time. (C) 2003 American Vacuum Society.