The properties of H-bonded systems of alkyl- and alkoxybenzene acids have been studied both experimentally (by IR-spectroscopy methods) and theoretically near the solid crystal – liquid crystal phase transitions. Partial dissociation of the cyclic dimers accompanied by formation of open chain-like associates and monomers is found. Their amount depends on the temperature and phaee of the sample. The theoretical model of rearrangement of H-bonds is suggested. The rate constant of the cyclic dimer – open associate transition is estimated.