A great interest to porous silicon (PSi) is mainly caused by it's photoluminescence properties. PSi can be described as self-intersecting quantum wires with varying cross section or just as an ensemble of nanocrystals. PSi was shown to be a photosensitizer of molecular oxygen under visible illumination. The oxygen molecules is being excited to singlet state while photosensitizing process. Since singlet oxygen (1O2) is a reactive oxygen species it can damage different parts of living cells and destroy them. 1O2 is widely used in biomedicine. For examle, the cancer photodynamic therapy is based on it's cytotoxic properties. Samples of microporous silicon were used in our research. They were formed by electrochemical etching in mixture of hydrofluoric acid and ethanol. Such samples are consisted of nanocrystals and pores with diameter about 3-4 nm. Porosity of the samples was about 60%. This data was obtained by x-ray diffraction and nitrogen adsorbtion at low temperatures. Fundumental aspects of interaction between silicon nanocrystals are well-investigated at cryogenic temperatures, but one should know system's behaviour at room temperature to apply it in medicine. The main problem was the detection of 1O2 in such ensembles at room temperature. This problem was solved for the first time by measurements of 1O2 luminescence with photon energy 0.98 eV. Fig. 1. Time dependency of integrated intensity of visible photoluminescence of porous silicon (solid line) and intensity of singlet oxygen luminescence (dashed line). The direct measurements showed relatively fast decrease of 1O2 concentration. Investigation of the mechanism of this decrease is the main purpose of this work. In our research visible photoluminescence of PSi and 1O2 were measured simultaneously (See fig. 1). This allowed us to find out relation between their intensities and control both processes of generation and relaxation of 1O2. Rate of generation 1O2 was found to decrease very fast just because of PSi photodegradation while rate of relaxation didn't drastically change. Kinetics of the photodegradation can be approximated using polynomial function with fractional exponent equal to -0.3. This fact points to stochastic dependency between elements of ensemble and even more to fractal nature of such nanosystems. All obtained results evidence of a big role of the oxygen diffusion in PSi. Since PSi is a dense material the interaction between it's surface and oxygen molecules should be essential. This leads to fast relaxation 1O2 to the triplet state and degradation of the silicon nanocrystals. An excited oxygen molecule can form a compound with silicon nanocrystal via chemosorption or oxidation. Electron charge displacement can enhance nonradiative processes in nanocrystal similar to Auger recombination process. This mechanism should be taken into account for biomedical applications of PSi. It seems that more promising to use isolated silicon quantum dots or particles produced by strong grinding of PSi, where the photodegradation will be significantly suppressed.