The hydride configurations in the hydrogenated amorphous silicon (a-Si:H) network have been studied by means of infrared absorption spectroscopy. The results on the film mass density of a-Si:H deposited by means of an expanding thermal plasma reveal the presence of two distinct regions in terms of hydrogen content and microstructure: below approximately 14 at. % H a-Si:H contains predominantly divacancies decorated by hydrogen, above 14 at. % H a-Si:H contains microscopic voids. These two distinct regions provide additional information on the origin of the low and high hydride stretching modes at 1980–2010 and 2070–2100 cm–1, respectively.