In-situ infrared spectroscopy is an especially powerful technique when applied to semiconducting electrodes. The use of a multiple-internal-reflection geometry brings an important improvement in sensitivity. Also, it is compatible with a cell geometry featuring low series resistance and allowing for quantitative studies in the presence of a Faradaic reaction and a fast modulation of potential, Additional interests include the possibility of analysing the changes in electronic absorption associated with the space-charge layer and/or electronic surface states, and the possibility of varying the polarisation of the infrared beam, giving extra information on the absorbing species. Here the potentialities of the technique are illustrated through an in-situ study of the modification of the hydrogenated silicon surface by anodic substitution of methyl groups in a Grignard electrolyte. This irreversible reaction is analysed by using a current pulse method. The electrochemical character of the reaction is demonstrated, and unambiguous indications on the reaction mechanism are obtained from the kinetics of modification. The electronic quality of the modified silicon surface is seen to depend on the halogen involved in the Grignard, which can be rationalised in terms of the reaction mechanism.