The effect of light-soaking, using a simulated daylight spectrum (colour temperature 5800 K) on the crystallinity, defect structure, and total hydrogen concentration of a-SM grown by HW-CVD is being investigated in an ongoing project. In this article, positron beam based electron momentum spectroscopy is applied to monitor the evolution of the open-volume defect structure after each illumination stage. The results indicate an initial increase in free volume at dangling-bond complexes on illumination, with no indication of a change in defect concentration or generation of larger microvoids. On further illumination, there is a reduction in the low electron momentum fraction, which is not accompanied by a similar change in the free-volume. This can be interpreted as a reconfiguration of the hydrogen in the dangling-bond complex, followed by its release into a mobile state. (C) 2003 Elsevier Science B.V. All rights reserved.