We present a systematic study of the local hydrogen bonding in hydrogenated polymorphous silicon thin films (pm-Si:H), a heterogeneous material deposited on the edge of crystallinity, by means of Fourier transform infrared spectroscopy. A vibrational mode at similar to2030 cm(-1) is reported and attributed to hydrogen atoms bonded in hydrogen-rich regions present at the interface between ordered regions and the amorphous matrix. This assignment is found to be in good agreement with previous spectroscopic ellipsometry and Raman spectroscopy studies. Combined with stress measurements we draw a picture of the nanostructure of pm-Si:H films, namely a two-domain material exhibiting: (i) large fluctuations of H content, (ii) a high mass density and (iii) the coexistence of highly strained crystalline phases with a relaxed amorphous matrix. Unexplained optoelectronic properties and metastability phenomena can also be accounted for by this picture.