A spectroscopic ellipsometer was integrated in a vertical furnace for measurement and control during chemical vapor deposition and thermal oxidation processes. The major goal of this activity was to adapt the ellipsometer arrangement to the furnace geometry with a minimum impact on the furnace process performance. Modifications in the furnace geometry were restricted as far as possible, to show that a fast integration in existing industrial equipment with minor costs can be done. This aim led to a novel beam-guiding system. This setup has been used for the in situ characterization of polysilicon formation, inside the vertical furnace during high temperature processes. The polysilicon-on-oxide structures were modeled using the Bruggeman effective-medium approximation (B-EMA). The crystallization process of amorphous silicon layers during annealing at 600 degrees C has been monitored. Layer thickness and the degree of crystallinity can be obtained simultaneously. The crystallinity can be described in the optical model using a 'mixture' of amorphous silicon and single-crystalline silicon in the B-EMA.