A new microscopic and kinetic model of light-induced metastability in hydrogenated amorphous silicon (a-Si:H) was recently proposed. Carrier recombination excites H from deep Si-H bonds into a mobile configuration, leaving a threefold-coordinated Si dangling bond (DB) defect at the site of excitation—a process long suspected to be an element of metastable DB production. Normally, mobile H are recaptured at DB defects and neither metastability nor net DB production results. However, when two mobile H collide, they form a metastable two-hydrogen complex and leave two-spatially-uncorrelated Staebler-Wronski DBs. The model leads to differential equations describing the evolution of the mobile H and DB densities and a variety of new predictions. New directions for improving the stability of a-Si:H are discussed. ©1999 American Institute of Physics.