The pH dependent structure of thymol blue (TB) in solution, a long-standing controversial issue, has been determined by quantum chemistry with an aid of multivariate analysis of electronic absorption spectra. It has been revealed that the TB structure varies from the neutral biprotonated form with opensulton ring showing red to the monovalent anion of the pure quinoid form showing yellow and then to the quinoidphenolate form showing blue on raising the pH for a solution.