The estimation of accurate activity coefficients and subsequent solubilities of amino acids/peptides in solution is fundamental for the design and optimization of efficient biotechnological separation and purification processes. In this work a new activity coefficient model is developed for amino acids and peptides in aqueous solution which comprises terms that represent nonidealities arising from short-range and long-range interactions: the former are taken into account by the UNIFAC method, and the latter are computed by the Debye-Huckel equation. The chemical equilibria leading to the formation of the zwitterionic, cationic, and anionic forms of the amino acids and peptides are also part of the new model. In the particular case of zwitterion, notwithstanding its null global charge, long-range contributions are also included due to its electrostatic dipole. The interaction parameters of the new UNIFAC groups are optimized using activity coefficients. compiled from the literature. The new model correlates and predicts experimental data with rmsd = 0.53% and 5.49%, respectively. The earlier UNIFAC-based models of Kuramochi et al., Pinho-Silva-Macedo, and Gupta-Heidemann reached 0.78%, 0.71%, and 7.38% for correlation, and 18.03%, 20.87%, and 13.28% for prediction, respectively. With respect to the isoelectric solubilities and the influence of pH upon solubility, the new model provides excellent behavior in the entire range of experimental conditions. The solubility model is explicit and very simple to implement.