The interaction of soluble chitosan (MW=15,000 Da, DD=85%, 0–0.1 wt%, 5mM phosphate buffer) with β-lactoglobulin (0.5 wt% β-lg, 5mM phosphate buffer) in aqueous solutions was studied at pH 3–7. Isothermal titration calorimetry (ITC) was used to provide information about the stoichemistry and enthalpy changes of the interaction. Electrophoretic and light scattering techniques were used to provide information about the electrical charge and aggregation of individual biopolymers and complexes. Biopolymer solubility was determined by measuring their concentration in the supernatant of aqueous solutions. ITC measurements showed that there was an exothermic interaction between chitosan and β-lg at pH 5–7, where the two biopolymers had opposite charges. The β-lg–chitosan interaction was most exothermic at pH 6 (≅−1560 kcal/mol of sugar units), with about one β-lg molecule bound per six chitosan sugar units at saturation. Light scattering measurements showed that 0.1 wt% chitosan was enough to reverse the electrophoretic mobility of β-lg from −0.8 to +0.8×10−8 m2 s−1 V−1 at pH 6, again indicating complex formation. Turbidity and solubility data indicated that an insoluble complex formed between chitosan and β-lg at pH 6 and 7. The results presented in this study should provide information that will facilitate the application of chitosan as a functional ingredient in food systems.