Silicon that is heavily doped with arsenic was treated in a microwave hydrogen plasma at temperatures of 530-600 degrees C. This hydrogenation increased the carrier concentration in the surface region where the concentration of As exceeded about 2 x 10(20) cm(-3), but not in the more lightly doped regions. X-ray photoelectron spectroscopy leads us to propose that As clusters are dissociated by atomic hydrogen and that this declustering increases carrier concentration.