A theoretical analysis of the mechanism of generation of singlet oxygen in the presence of photoexcited nanoporous silicon is presented. It is demonstrated that the mechanism of generation of singlet oxygen is based on nonradiative energy transfer from nanoporous silicon to an oxygen molecule by the exchange mechanism. An analytical expression of the probability of energy transfer from nanoporous silicon to an oxygen molecule is obtained, and a numerical estimate of this process is given. The numerical estimation is of the order of 10^3–10^4 s^–1, a value that agrees rather well with the experiment.