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The thermodynamics of the interactions of different ellagitannins with two proteins, namely bovine serum albumin (BSA) and gelatin, were studied by isothermal titration calorimetry. Twelve individual ellagitannins, including different monomers, dimers and a trimer, were used. The studies showed that several structural features affected the interaction between the ellagitannin and the protein. The interactions of ellagitannins with proteins were stronger with gelatin than with BSA. The ellagitannin-gelatin interactions contained both the primary stronger and the secondary weaker binding sites. The ellagitannin-BSA interactions showed very weak secondary interactions. The ellagitannins with a glucopyranose core had stronger interaction than C-glycosidic ellagitannins with both proteins. In addition, the observed enthalpy change increased as the degree of oligomerization increased. The stronger interactions were also observed with free galloyl groups in the ellagitannin structure and with higher molecular flexibility. Other smaller structural features did not show any overall trend.