Fourier transform infrared attenuated total reflection (FTIR-ATR) spectroscopy has been used to analyse the in-situ adsorption of alkyl and perfluoroalkyl trichlorosilane molecules onto silicon substrates. These results have been analysed to calculate the Gibbs surface excess of an adsorbed surfactant. This data was then used to calculate the area per molecule occupied by n-octadecyltrichlorosilane, (OTS), and by perfluorodecyl(ethyl) trichlorosilane (F8) films on the silicon ATR prisms. The in-situ results for the area per molecule occupied by OTS and F8 have been compared with data on similar systems, using X-ray diffraction and atomic force microscopy. Our results were found to agree well with the X-ray diffraction results but not with those obtained from atomic force microscopy (AFM). Similar studies, in which the silicon or glass substrates were removed from solution prior to investigation, were carried out using FTIR-ATR spectroscopy and AFM. These ex-situ FTIR-ATR and AFM results, whilst complementing each other, showed differences from the in-situ FTIR-ATR results probably due to the different experimental conditions employed.