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The aim of the present study was the investigation of the isomerization and epimerization kinetics of the aspartyl tetrapeptide Gly-Phe-Asp-GlyOH at alkaline conditions. Incubations of the model tetrapeptide in sodium borate buffer, pH 10 and ionic strength 0.2 M, at 25 °C and 80 °C were analyzed by a validated CE–UV assay and fitted according to a pharmacokinetic model. CE–ESI-MS was used for peptide identification. Enantiomerization and isomerization of the aspartyl residue of the model tetrapeptide was observed under all experimental conditions applied. Differences in the velocity and the ratios of the rates of the degradation reactions indicated different effects of temperature on the individual reactions. At 80 °C, a rapid formation of β-Asp and d -Asp containing isomers from Gly- l -Phe-α- l -Asp-GlyOH was monitored. Rate constants of the hydrolysis of the succinimide (Asu) intermediate generally exceeded the formation of the intermediate from α/β-Asp peptides. A higher rate constant was observed for the enantiomerization from l -configured Asu compared to d -Asu. At 25 °C, epimerization and isomerization equilibrium was not reached within 5208 h. Compared to 80 °C different ratios of the individual reaction rates were noted. Moreover, inversion of the sequence of the first 2 amino acids was noted as a minor side reaction at 80 °C.