We present experimental evidence that the surface passivation of crystalline silicon (c-Si) by hydrogenated amorphous silicon (a-Si:H) is metastable. Photo-conductance decay measurements of the effective minority carrier lifetime in c-Si show that the surface recombination velocity of the carriers at the c-Si/a-Si:H interface is reduced by annealing at temperatures up to ∼ 350°C, but relaxed to higher values after further thermal treatment. The relaxation is thermally activated and faster at higher temperatures. We attribute this phenomenon to the thermal equilibration of charged defects in a-Si:H. Our finding suggests that a-Si:H passivation may require thermal stabilization for realizing reliable photovoltaic devices. © 2011 IEEE.