A technique for in situ monitoring of changes on surfaces of semitransparent thin films is presented. This technique combines ellipsometry and total internal reflection and is called total internal reflection ellipsometry. It utilizes the high surface sensitivity of ellipsometry and can be applied to detect and quantify very small changes on thin film surfaces. One example on an application is corrosion monitoring. The main advantage in comparison to ordinary ellipsometry is that measurements are done from the "back side'' of the sample and thus the probe beam does not propagate through and will not be influenced by the media reacting with the surface. An overview of total internal reflection ellipsometry and results from corrosion monitoring on thin copper films are presented. (C) 2003 The Electrochemical Society. All rights reserved.