Acidity constants of benzotriazolium salts and the corresponding ylides represent quantitative parameters for the estimation of their stability. Experimental pKa values were determined using UV-spectrometry. For each species involved in acid-base equilibria, the theoretical total energies Et, solvatation energies Es and UV-VIS electronic transitions were calculated by AM1, PM3, RHF/6-31G*, AM1/H2O and ZINDO methods, respectively. Theoretical parameters confirm the experimental pKa values. Acidity constants together with theoretical energies constitute critical parameters for achieving good yields for the synthesis of ylides from their corresponding benzotriazolium salts
