The material properties of hydrogenated amorphous silicon (a-Si:H) have been known to change when exposed to elevated temperatures. In this work we report on the thermal stability of the structural disorder, hydrogen content and defect structure of hot-wire deposited a-Si:H when exposed to temperatures in excess of 100 degrees C. Prior to thermal hydrogen diffusion a change in the defect structure is observed, caused by the creation of low concentrations of microvoids. There is evidence of vacancy clustering at 400 degrees C, caused by the alignment of unterminated Si dangling-bonds that consequently results in an increase in the defect size, concentration or both. Raman scattering shows evidence that no crystallization is induced at 400 degrees C and that the structural disorder increases upon annealing. (c) 2005 Elsevier B.V. All rights reserved.