A comprehensive DTA study is reported of 17 halophenols and of the corresponding pnitrobenzoylchloride 3,5-dinitrobenzoylehloride and p-phenylazobenzoylchloride derivatives, prepared ~in-situ'by heating intimate mixtures of phenol and acid chloride in the DTA system. The thermal analysis data, in particular, the derivative formation temperatures, are interpreted in terms of the inductive, mesomeric and sterie effects associated with the ring substituents of the phenol and acid chloride and the extent of inter- and intramolecular hydrogen-bonding existing in these systems. The DTA data collectively provide a reference base for the identification and characterization of halophenols via thermal analysis. Phenols are of primary concern as environmental pollutants. These compounds are components of numerous commercial products such as pesticides [1] wood preservatives [2, 31 fungicides [4, 5] disinfectants [6] and dyes [7, 8]. Further, phenols are often by-products of industrial processes, such as coke ovens and tar distilleries [9, 10] petrochemical industries [11, 12] kraft pulp mill bleaching effluents [13, 14] and coal liquefaction plants [aSl. Analytical methods available for the characterization and determination of phenols in the solid state are few in number. Infrared [16-19] and photoelectron spectroscopy [20] have been applied to structure evaluation of numerous phenols; however, classical spectroscopic techniques have limited applicability to the characterization of the components of a phenol mixture, due to overlap of the characteristic spectral bands. The old, familiar characterization procedure of derivative formation and determination of the melting point thereof, has been applied to phenols.