Adsorption and diffusion of Si atoms on a hydrogen-terminated Si(0 0 1)-2 x 1 surface is studied using scanning tunneling microscopy/spectroscopy and first-principles total-energy calculations. We find that the Si atoms randomly adsorb at the bridge site of Si dimers forming SiH2 clusters at room temperature, and move into the off-centred inter-bridge site after annealing at 250 degreesC. (C) 2003 Elsevier Science B.V. All rights reserved.