The decomposition of H₂O and D₂O on silicon surfaces was studied using transmission Fourier-transform infrared (FTIR) spectroscopy. These FTIR studies were performed in situ in an ultrahigh vacuum chamber using high surface area porous silicon samples. The FTIR spectra revealed that H₂O (D₂O) initially dissociates upon adsorption at 300 K to form SiH (SiD) and SiOH (SiOD) surface species, i.e., H₂O → SiH + SiOH. The decomposition of these surface species was then monitored using the Si---H (Si---D) stretch at 2090 cm⁻¹ (1513 cm⁻¹), SiO---H (SiO---D) stretch at 3680 cm⁻¹ (2707 cm⁻¹) and the Si---O---Si stretch at 900–1100 cm⁻¹. As the silicon surface was annealed to 650 K, the FTIR spectra revealed that the SiOH surface species progressively decomposed to Si---O---Si species and additional SiH species, i.e., SiOH → SiH + SiOSi. Above 650 K, the SiH surface species decreased concurrently with the desorption of H⁻¹ from the porous silicon surface. New blue-shifted infrared features in the Si---H stretching region were observed at 2119, 2176 and 2268 cm⁻¹ after annealing above 600 K. Additional infrared studies of partially hydrogen-covered porous silicon surfaces exposed to O₂ suggested that these blue-shifted Si---H stretching vibrations were associated with silicon surface atoms backbonded to one, two or three oxygen atoms, respectively.