The infrared reflection spectroscopy supported by spectral simulation is employed in the characterization of the structure of chemically adsorbed thin organic films on metallic and semiconductor substrates. The strategy for recording a good quality spectra at submonolayer coverage, and understanding of the changes in reflection spectra resulting from the nature of bondings, surface perturbation, orientation and optical effect are discussed. The reflection spectra of thin films adsorbed on high-absorption (metallic) and low-absorption (semiconductor) substrates show different optical effects, therefore the theoretical background and experimental results are reviewed for these two types of substrates separately. Examples of the investigation of thin films, which involve ex-situ and in-situ studies are reviewed, demonstrating the kind of insight such spectral data can provide. Generally, good agreement was obtained between experimental and theoretically calculated results which allowed to describe the structure of the adsorbed thin films on copper and cuprous sulfide. Supporting results obtained from other spectroscopic and electrochemical methods are also discussed.