Protonation equilibria of L-proline and L-valine in varying compositions (0.0-60.0% v/v) of 1, 2-Propanediol-water mixtures were investigated pH-metrically. Titrations were performed at 303.0 K and the ionic strength of the medium was maintained at 0.16 mol L-1 using sodium chloride. The best fit chemical models of the protonation equilibria were based on crystallographic R-factor, Χ2, skewness, and kurtosis. All the protonation constants of proline and valine increased with increasing propanediol content. This is attributed to the dielectric constant of the medium.