Monomer-dimer equilibrium of nitroso compounds was investigated in the solid state under cryogenic photochemical conditions. It was found that nitroso dimers can be UV-photolytically converted to nitroso monomers at 12 K, and reverted to dimers by visible light irradiation or warming above 170 K. Such a photothermal "chemical switch", by which it is possible to break the chemical bond between two nitrogen atoms and bind them again, could eventually be used in supramolecular self-assembly systems. The reaction is very efficient and controlled by topochemical factors. This phenomenon was studied spectroscopically in a series of cyclic and polycyclic nitroso compounds.