Department of Chemistry, University of Jordan, Amman, Jordan Manuscript received 25 March 1980, accepted 27 July 1980 The apparent heat of proton ionization, ��� H', for phenolphthalein in aqueous solutions containing 6 per cent ethanol by weight was determined by two methods, (a) the conventional spectrophotometric method in which apparent dissociation constants are obtained as a function of temperature and (b) the novel thermochromic method. The reasonably good agreement between the results of the two methods establishes the latter as a powerful and unique tool for obtaining ��� H' values of weak acids.