2-Hydroxybiphenyl demonstrates proton transfer in the lowest excited singlet state in alcohol/water mixtures as well as in pure water. The dissociation reaction of the directly excited neutral molecule is strongly solvent dependent and independent of solution acidity. The reprotonation of the conjugate base, however, depends predominantly on the acidity of the solution and only on the bulk dielectric constant of the solvent. The linear relationship between the ratio of the relative fluorescence efficiencies of 2-hydroxybiphenyl and its conjugate base and the hydrogen ion concentration is obtained only if proper Bronsted activity factors are included in the relationship. These factors can be calculated from classical electrostatics and are simple powers of the activity coefficients that would be derived from Debye-Hückel theory and which are necessary to convert the formal hydrogen ion concentration to hydrogen ion activity. With this approach it is possible to calculate pH from hydrogen ion concentration in alcohol/water solutions containing mole fractions of methanol, ethanol, isopropyl alcohol, and t-butyl alcohol up to 0.5, 0.3, 0.1, and 0.05, respectively.