We report the effect of a N-methylpyridinium substituent on the photoacidity enhancement of a phenol derivative. The steady-state and time-resolved fluorescence of this new phenolic 'super' photoacid (HPPy) are measured in different solvents in comparison with its methoxy analogue (MPPy). Solvatochromic shifts and nonradiative rate constants ascribable to excited state proton transfer ( ESPT) are correlated with the empirical Kamlet - Taft parameters (pi*, alpha, and beta). Whereas, spectral shift of MPPy is mainly correlated to pi* parameter, HPPy is also strongly influenced by beta. At 77 K, ESPT is hampered and leads to the switch 'on' emission of conjugated base. (C) 2008 Elsevier B.V. All rights reserved.