The behavior of hydrogen in a-Si: H during laser crystallization and rapid thermal annealing (RTA) has been investigated. High quality crystallized films have been obtained on fused quartz plates by cw Ar + laser scanning. However, poor quality films are obtained on SiO 2/Si substrates because of the generation of bubbles and holes. A hydrogen content of 11 at.% (5.5 × 10 21 cm -3) has been measured in as-deposited a-Si: H films. For the case of fused quartz substrates the hydrogen content decreased to 2 × 10 20 cm -3 during the crystallization process and decreased further to 6 × 10 19 cm -3 after 35 s RTA at 1150°C. The remaining hydrogen is favorable for passivation of defects and grain boundaries.