We studied the phase behavior of a model binary mixture of eye lens crystallin proteins using first-order thermodynamic perturbation theory. The instability boundary, or spinodal surface, was found to be very sensitive to the strength of the attraction between the two proteins, and also to respond to this interprotein attraction strength in a non-monotonic fashion. In particular, in the case of either weak or strong attractions, these eye lens solutions become thermodynamically unstable. Interestingly, attraction strengths that correspond closely to those of proteins isolated from the living lens fall right within the stable region of the phase diagram. This non-monotonic stability suggests new molecular mechanisms for eye lens opacification in cataract.