Ellipsometry is a sensitive and noninvasive technique for the characterization of thin films. A recently developed ellipsometer, based on the four-detector photopolarimeter, was arranged outside a UHV chemical vapor deposition chamber for the in situ monitoring of film growth processes. The instrument showed a sensitivity in the submonolayer range when used to follow the growth of germanium thin films deposited on silicon substrates. As the main instrument drawback is represented by the need to have precise alignment, an effective positioning procedure was developed to obtain a positioning error smaller than 0.1°.