Ellipsometric microscopy is a novel technique that combines the merits of ellipsometry and light microscopy, i.e. it allows noninvasive, label-free measurements of thin film thickness and refractive index at high lateral resolution. Ellipsometric microscopy has been successfully applied to silicon-air interfaces. However, typical biological systems require immersion in an aqueous buffer. Thus the authors have adapted the instrument for the observation of the interface between glass and water. In particular, the comparatively small differences in refractive indices between substrate and ambient media proved to be a challenge for instrument design. The first experiments with this new instrument are presented.