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At the altitudes of the mesosphere – lower thermosphere, OH and HO2 play a significant role in many physicochemical processes. Thus, monitoring of their spatiotemporal evolution together with other chemically active trace gases is one of the most important problems for this atmosphere region, in which direct measurements are difficult. The paper studies the nighttime OH and HO2 chemical equilibria using the 3D chemical transport modeling within the general approach including the extraction of the main sources and sinks in the equilibrium space-time areas and derivation of analytical criteria for equilibrium validity. The presented analysis shows that there are extended areas, where nighttime HO2 and OH are close to their local equilibrium concentrations determined mainly by the reaction between HOx – Ox components themselves and with H2O2, N, NO, NO2, and CO. In the upper mesosphere – lower thermosphere, the equilibrium expressions can be shortened, including the HOx – Ox chemistry only. These conditions describes the HO2 and OH equilibrium from the top to the some lower borders, the altitude position of which vary in the interval between 72–73 and 85 km and depends essentially on season and latitude. The developed analytical criteria almost everywhere well reproduce the main features of these borders. The obtained results allow to extend the abilities of previously proposed methods for the retrieval of poorly measured components from measurement data and to develop new approaches. [ABSTRACT FROM AUTHOR]