A comprehensive DTA study is reported of ten polyhydroxy-phenol and eight naphthols and of the corresponding p-phenylazo benzoylchloride, p-nitrobenzoylchloride and 3,5-dinitrobenzoylchloride derivatives, prepared «in situ» by heating intimate mixtures of phenol and acid chloride in a conventional DTA system. The thermal analysis data and,in particular, the derivative formation temperatures, are interpreted in terms of the inductive, mesomeric and steric effects associated with the phenol and acid chloride and the extent of inter- and intramolecular hydrogen-bonding existing in these systems