On adsorption of various vapours, the electrical conductivity of nitroaromatic semiconductors 9-nitroanthracene, 1,4-dinitronaphthalene and 1,3,5-trinitrobenzene changes appreciably. This phenomenon has been used as a probe to study the adsorption and desorption processes in these nitroaromatic polycrystals. The adsorption and desorption kinetics have been found to follow the modified Roginsky-Zeldovich relation. Two adsorption processes have been identified. One, a two-stage process, the first of which gives a Lennard-Jones potential energy curve and is followed by a rate-determining transition over a potential energy barrier to the second stage of adsorption forming weakly bound complexes between the vapour molecules and the nitroaromatic crystallites. In the second process, in addition to the two stages of the first process, a third stage with a comparatively deep potential energy surface is reached by activation over a second potential barrier and strongly bound complexes are finally formed.