Thermal annealing of porous silicon at 400 and 450 C leads to the desorption of hydrogen from the SiH2 sites and to the quenching of the photoluminescence. After exposure for 2 months to ambient air at 300 K, the photoluminescence is restored and becomes more intense than that of the as-etched sample. IR spectrometry and thermal desorption spectrometry show that it is the result of the passivation of the porous silicon surfaces by oxygen and water vapor.