We have studied the interaction of hydrogen with thermal donors in silicon using transient capacitance and current spectroscopy. We find that a large degree of thermal donor passivation (a factor of 40) can be achieved by hydrogen plasma exposure at 120 °C. The residual electrical activity is shown to arise from perturbedE(0.07) andE(0.15) donor states. Annealing at 200 °C almost completely reactivates the low concentration of thermal donors present in these samples. A model involving different incorporation sites for hydrogen is proposed to explain the results.