Aqueous two-phase systems are discussed for the extraction of biomolecules like, for example, amino acids, peptides and enzymes from aqueous phases. The development of such extraction processes requires experimental results for the partitioning of model substances, as well as methods for correlating and predicting such phase equilibria. The present contribution reports experimental results for the partitioning of small amounts (about 1 mg/g) of amino acids glycine, L-glutamic acid, L-phenylalanine and L-lysine and some of their low molecular peptides in aqueous two-phase systems of dipotassium hydrogen phosphate and poly(ethylene glycol) of molecular mass of about 6000 and 35 000 at about 293 K. The experimental results for the partitioning of the amino acids and some of their peptides are used to determine interaction parameters of a group contribution model for the excess Gibbs energy. The remaining experimental data are used to test the model for predicting partitioning of peptides in PEG/K2HPO4 two-phase systems.