In this article, a new group contribution model is suggested for obtaining the thermodynamic properties of biomolecules in aqueous solutions. Accordingly, a Freed-FV model has been applied for the combinatorial free-volume term. The activity coefficients, solubilities, densities, and vapor pressures of amino acids and simple peptides in aqueous solutions were correlated, using the proposed group contribution model. Group interaction parameters of the proposed model were obtained by use of experimental data from amino acids available in the literature. The results demonstrate that the group contribution model can accurately correlate activity coefficient, solubility, density, and vapor pressure data for aqueous amino acid and peptide solutions. Furthermore, the osmotic pressures of aqueous solutions containing lysozyme and ammonium sulfate were calculated at ionic strengths of I and 3 M and pH values of 4, 6, and 8. The results obtained from the suggested model were compared with those obtained from the virial osmotic model. Also, the model was coupled with the Debye-Hockel model to correlate partition coefficients of biomolecules in polymer-salt aqueous two-phase systems. The results showed that the new group contribution model can accurately correlate partition coefficients of biomolecules in polymer-salt aqueous two-phase systems.