We propose a combination of the spectroscopy with infrared radiation emitted by a free-electron laser (FEL) and of a scanning near-field optical microscope (SNOM) to investigate one of the major problems in today's solid state physics: lateral variations of solid interface properties. This approach enabled us to measure the local optical properties of a buried PtSi/Si interface, of diamond grains on top of a silicon surface, of GaAs/AlGaAs wells and of an integrated microcircuit. The reflectivity in the SNOM images revealed features that were not present in the corresponding shear-force (topology) images, and which were due to localized changes in the bulk properties of the sample. The size of the smallest detected features clearly demonstrated that near-field conditions were reached: an optical spatial resolution well below the diffraction limit was observed.