The stochiometric protonation constants of (log(10) K (1) and log(10) K (2)) some alanyl dipeptides (Ala-Tyr, Ala-Phe, Ala-Val, Ala-Leu, Ala-Ala and Ala-Met) were determined in binary mixtures of water with 1,4-dioxane and dimethylsulfoxide containing 20, 40 and 60 % (v/v), using a potentiometric titration method at 25 +/- A 0.1 A degrees C and constant ionic strength of 0.10 mol center dot L-1 sodium chloride. The autoprotolysis constant values (K (ap)) of the media were also determined for each solvent mixture. The protonation constants of the dipeptides were calculated with the computer program BEST and selection of the best fit chemical models was based on the statistical parameters. The effects of the different amino acids bound to the alanine on the acidity of the alanyl dipeptides have been investigated. Finally, the results are discussed in terms of the effect of the solvent composition on the protonation constants.