Thermodynamic acidity constants have been measured over the temperature range 5-50° for aqueous solutions of sodium 4?-dimethylaminoazobenzene- 4-sulphonate (methyl orange) and sodium 4?-dimethylaminoazobenzene-2- sulphonate (ortho-methyl orange). From these data values of the standard enthalpy, entropy, and heat capacity changes have been calculated for these compounds. These results are discussed in conjunction with previous spectrophotometric and other data with reference to the nature of the equilibrium systems involved in these protonation reactions. It is concluded that existing evidence does not allow an unequivocal assignment of the sites of protonation of these and related molecules.