Various recent applications of infrared ellipsometry (IR-SE) are reviewed with a particular emphasis on in situ studies. It is shown from the characterisation of hydrogen terminated c-Si surfaces that IR-SE can achieve submonolayer sensitivity. As compared to UV-visible SEI the IR range provides vibrational sensitivity. It is shown that a very precise determination of thin film composition can be obtained from the dependence of vibrational properties on the local environment. The capability of probing vibrational properties constitutes a crucial advantage for studies in which chemical information is needed, such as polymer interfaces or surface treatments. From a more fundamental point of view, IR-SE can also probe the local structural order of a thin film material, such as hydrogenated amorphous silicon. As a consequence, IR-SE appears as a promising characterisation technique for surfaces and thin films.