In this paper, we describe the interaction of CD3CN and pyridine (used as probes of Lewis acid centers) with TS-1 catalyst. Interaction of CD3CN and pyridine with Ti(IV) centers leads to surface adducts characterized by well-defined IR spectra which can be distinguished from those ascribed to interaction with internal hydroxyl groups, which are the most abundant acidic species present in TS-1. In both cases, spectroscopic features were observed which are characteristic of TS-1 and completely absent in pure siliceous silicalite-1. This IR study has demonstrated that Ti(IV) centers embed-led in the MFI framework have a Lewis acidity strength comparable of that of Ti(IV) sites at the surface of TiO2. The spectroscopic results have been compared with computational data obtained on cluster models on the basis of the ONIOM approach.