The optical constants in the infrared spectral range and the thickness of a surface layer are simultaneously determined by reflection based spectroscopic infrared ellipsometry. In the past experimental progress has been used to increase sensitivity with the aim to detect ever thinner layers. Reaching the monolayer limit by now, methodic efforts focus on revealing structural details such as anisotropy and lateral heterogeneity caused primarily by molecular orientational order. The basis of the method and present methodical approaches are outlined. Aspects of using synchrotron radiation for infrared ellipsometry and of setting up an infrared beamline are discussed.