The main aim of this work is introducing a novel protic acidic ionic-liquid catalyst for organic synthesis. Thus, N,N-diethyl-N-sulfoethanaminium methanesulfonate ([Et3N-SO3H][MeSO3]) was synthesized by the reaction of triethylamine with chlorosulfonic acid, and then with methanesulfonic acid. The structure of this novel protic acidic ionic liquid was identified by FT-IR, 1H NMR, 13C NMR and mass spectral data. Its catalytic activity was examined for the solvent-free reaction of 2-naphthol with arylaldehydes to provide 14-aryl-14H-dibenzo[a,j]xanthenes. [Et3N-SO3H][MeSO3] efficiently catalyzed the reaction, and the products were obtained in high to excellent yields and in short reaction times. KEY WORDS: Protic acidic ionic liquid, N,N-Diethyl-N-sulfoethanaminium methanesulfonate ([Et3N-SO3H][MeSO3]), 2-Naphthol, Arylaldehyde, 14-Aryl-14H-dibenzo[a,j]xanthene Bull. Chem. Soc. Ethiop. 2019, 33(1), 69-76DOI: https://dx.doi.org/10.4314/bcse.v33i1.7