Buffered aqueous two-phase systems are effective extraction systems for separating amphoteric hydrocarbons like, for example, polypeptides from aqueous phases. The design and basic engineering of such processes require the knowledge of the liquid-liquid equilibrium. Methods to calculate and predict the liquid-liquid phase equilibrium in those systems can only be developed when a large data base is available. The present contribution is aimed at that goal by presenting experimental results for the partitioning of small amounts (approximate to 0.001 g/g solution, of model substances-amino acids glycine, L-glutamic acid, L-phenylalanine, and L-lysine as well as dipeptides and some higher, but still low, molecular peptides of those single amino acids-in aqueous two-phase systems of dextran and poly(ethylene glycol) in the presence of small amounts (about 0.05 mol/kg) of K2HPO4/KH2PO4-buffer at about 293 K. A group contribution model for the excess Gibbs energy is used to correlate/predict that liquid-liquid phase equilibrium.