The effect of temperature on the electronic absorption spectra of methyl orange indicator was investigated at different concentrations of the non-ionic surfactant Triton x-100. The change in the enthalpy of ionization of the indicator is obtained at different [TX-100] concentrations. When the dye molecules are all solubilized into the micellar core, the enthalpy change is approximately zero. The existence of different indicator ionic species in equilibrium is explained from a thermodynamic point of view.