To gain more information about the effect of solvent on 4,5-dihydro-1H-1,2,4-triazol-5-ones, the stoichiometric protonation constants of thirteen triazoles in ethanol-water mixtures were determined at an ionic strength of 0.10 M NaCl and at 25.0 +/- 0.1 degrees C under nitrogen atmosphere. A potentiometric method was used and the calculation was carried out using the PKAS computer program. The correspondingpK(a) values of these triazoles were determined in ethanol-water mixtures. Thus, the effects of solvent and molecular structure upon acidity were investigated. The logarithm of the protonation constants of the title 4,5-dihydro-1H- 1,2,4-triazol-5-ones decreased linearly with increasing ethanol content, but the values determined with 80% ethanol did not follow this linear trend. The variation of these constants is discussed on the basis of specific solute-solvent interactions.