Porous silicon lattice deformation and its evolution with storage time are the main subjects of the present investigation. Transmission electron microscopy, x-ray diffractometry, and electronic paramagnetic resonance studies are reported. Silicon wafers of 0.01 Omega cm resistivity n(+)-type and (111) orientation were used as starting material. The porous silicon layers of 1.8 g/cm(3) volume density were formed by anodization in 12% HF aqueous solution at current density of 20 mA/cm(2). The as-grown porous silicon layers were stored in air atmosphere for a long time and were subjected to heat-treatment in vacuum. Annealed porous silicon samples were stored again in air or in controlled atmosphere of hydrogen, nitrogen, or oxygen. The observed behavior of the porous silicon lattice parameter during storage in air and during thermal treatment allowed us to suppose that absorption/desorption processes, taking place at the inner developed porous silicon surface, are responsible for the lattice deformation.