Fiber optic reflectance spectroscopy (FORS) in the entire 380 11,000 nm range has been used to non-invasively characterize Leonardo's painting Madonna of the Yarnwinder. A preliminary analysis of the recorded spectra made it possible to obtain useful information concerning the pigments used, previous restoration work, the preparatory layer, and the binding medium. In particular, it was found that Thenard's blue was used in a previous restoration instead of the original ultramarine. As regards the painting technique, typical spectral features of oil binding medium are present in the mid-IR spectra, while near IR spectra reveal the occurrence of clay in the preparatory layer. -- MCI