ABSTRACT: The kinetics of an acid-base catalyzed reaction, aspartame degradation, were examined as affected by the changes in pH and pKa values caused by adding polyols (sucrose, glycerol) to phosphate buffer. Sucrose-containing phosphate buffer solutions had a lower pH than that of phosphate buffer alone, which contributed, in part, to reduced aspartame reactivity. A kinetic model was introduced for aspartame degradation that encompassed pH and buffer salt concentrations, both of which change with a shift in the apparent pKa value. Aspartame degradation rate constants in sucrose-containing solutions were successfully predicted using this model when corrections (that is, lower pH, lower apparent pKa value, buffer dilution from the polyol) were applied. The change in buffer properties (pH, pKa) from adding sucrose to phosphate buffer does impact food chemical stability. These effects can be successfully incorporated into predictive kinetic models. Therefore, pH and pKa changes from adding polyols to buffer should be considered during food product development.