The effect of gaseous and liquid nitrogen dioxide on the composition and electronic properties of porous silicon (PS) is investigated by means of optical spectroscopy and electron paramagnetic resonance. It is detected that the interaction process is weak and strong forms of chemisorption on the PS surface, and the process may be regarded as an actual chemical reaction between PS and NO2. It is found that NO2 adsorption consists in forming different surface nitrogen-containing molecular groups and dangling bonds of Si atoms (P-b-centers) as well as in oxidizing and hydrating the PS surface. Also observed are the formation of ionic complexes of P-b-centers with NO2 molecules and the generation of free charge carriers (holes) in the volume of silicon nanocrystals forming PS.