A molecular crystal of 6-[2-propyl-4-(4-pyridylazo)phenoxy]hexanoic acid prepared by sublimation showed two thermally reversible transitions at 17 and 172 degrees C on heating. Temperature dependence of its X-ray diffraction profile analyzed on the basis of its crystal structure revealed that the reversible transitions were related to a thermally induced hierarchical structural transformation involving cleavage of a C-H center dot center dot center dot O hydrogen bond, followed by cleavage of a O-H center dot center dot center dot N hydrogen bond.