Changes of midgap‐state profiles of light‐soaked undoped hydrogenated amorphous silicon are measured in the process of a 150 °C annealing by transient heterojunction‐monitored capacitance measurements. Monomolecular annealing kinetics are found to be suitable for explaining the results, and the thermal activation energy for annealing is determined at each energy position of midgap states. This activation energy decreases with an increase in the energy position measured from the conduction‐band edge.