Concentrated γ-crystallin and lysozyme solutions have been reported to undergo a fluid–fluid phase separation when cooled below a critical temperature. This behavior is under control of the weak forces acting in solution between macromolecules. We have used small angle x-ray scattering at the synchrotron radiation facility LURE (Orsay, France) to analyze the interaction potentials. A model of attractive interactions which depends upon three parameters, protein diameter, potential depth, and range, is able to account for both the x-ray structure factors measured at high temperature and for the low temperature phase separation. Although van der Waals forces could be at the origin of the attractive interaction potentials, other more specific effects also contribute to the protein phase diagrams.