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A first study by in situ FTIR spectroelectrochemistry of TTF and TTM-TTF has been undertaken. The oxidation, in this case, is caused only by electrochemistry, which constitutes a clear advantage over chemical oxidation since no side products are present in the solution. In this context, we obtained the signals of neutral, radical cation, and dication species of TTF and TTM-TTF. The experimental conditions were chosen in order to avoid the possible formation of p-dimer species and to obtain a satisfactory signal-to-noise ratio. A weak signal was detected for TTF and a stronger one for TTM-TTF. The changes induced by the oxidation process in the IR spectra of TTF and TTM-TTF have been analyzed with the aid of B3P86/6-31G** density functional theory (DFT) calculations, which allows for a comprehensive assignment of the bands observed. DFT calculations show that the IR signal associated to the asymmetric stretching of the lateral C=C bonds can be used as a structural signature to identify the different oxidation states of TTF and TTM-TTF. The frequency downshift and the intensity increase undergone by the n asym (C=C) vibration upon oxidation are shown to be the key factors to understand the evolution of the IR spectra.