Interaction between ozone molecules and silicon nanocrystallites in porous silicon (por-Si) results in effective luminescence similar to optically excited emission but the mechanism still is not quite understood. To study the process the kinetics of both adsorption and oxidation of highly porous silicon surface under exposure to ozone (¹⁶O₃ and ¹⁸O ₃) at low temperatures were measured using FTIR- and luminescence spectroscopy. It is shown that ozone chemisorption on por-Si surface takes place yet at liquid oxygen temperature. The oxide coat formed instantly after ozone admission inhibits further oxidation and results in the predominance of ozone physisorption.