A study of thermal annealing of a-Si:H samples between 300 and 600°C has been carried out. At increasing annealing temperatures, the sub-gap absorption measured by PDS increases showing two inflections, centered at 375 and 550°C. The hydrogen content measured by thermal desorption spectroscopy evolves in the same temperature range, whereas the evolution of the hydrogen content deduced from the IR transmission spectra differs, decreasing sooner and vanishing already at about 450°C.