The interaction of NO2 with mesoporous silicon (m-PS), obtained by the electrochemical etching of p(+)-type crystalline silicon in HF solutions, has the nature of adsorption on definite surface sites, involving most of the boron atoms present in the sample, brought to the surface by the etching mechanism. Adsorption brings about the release of hole carriers to the solid, which absorb in the IR according to the Drude model for a free-charge-carrier gas: this renders the phenomenon measurable, notwithstanding the very low concentration of surface sites. Adsorption follows a Langmuir isotherm; that is, the involved sites are apparently equal and noninteracting. This, as well as the insensitivity of IR modes of surface SiHx species to the increase in carrier concentration, strongly indicates that the outermost layers of m-PS crystallites have an insulator behavior. Estimation of the enthalpy of adsorption suggests that a chemisorption process is involved: this is confirmed by the kinetics of desorption being lower than that of adsorption.