The effects of vacuum annealing on the optical absorption spectra in the visible and infrared ranges, photoluminescence intensity, and concentration of paramagnetic centers in free‐standing porous silicon films were investigated in a temperature range of 100–600 °C. It was found that heat‐induced hydrogen desorption decreased the porous silicon band gap, which suggests that band‐gap energy depends on hydrogen coverage of nanoparticles. The annealing also leads to increasing concentration of defects that were identified as silicon dangling bonds. The energy distribution of the dangling‐bond states was estimated from the absorption spectrum. ©1996 American Institute of Physics.