Hydrogen chemistry in thin films and biological systems is one of the most difficult experimental problems in today's science and technology We successfully tested a novel solution, based on the spectroscopic version of scanning near-field optical microscopy (SNOM). The tunable infrared radiation of the Vanderbilt free electron laser enabled us to reveal clearly hydrogen-decorated grain boundaries on nominally hydrogen-free diamond films. The images were obtained by SNOM detection of reflected 3.5 mum photons, corresponding to the C-H stretch absorption, and reached a lateral resolution of 0.2 mum, well below the lambda /2 (lambda = wavelength) limit of classical microscopy.