Dysregulation of pH is one of the hallmarks of solid cancerous tumors. An increase in extrusion of acid from tumor cells leads to acidification of the extracellular environment, which in tumors is 0.4 to 0.8 pH units lower than in normal cells. The efficiency and selectivity of proteins (e.g. toxins and antibodies) designed to identify and kill tumor cells would be enhanced if they could be engineered to function as pH switches that are activated/deactivated by small changes in pH in the physiological range. We have demonstrated that buried ionizable residues (Lys, Glu and Asp) are useful for this purpose. Most buried ionizable residues titrate with anomalous pKa values (depressed for Lys and elevated for Glu/Asp) simply by virtue of being buried. This ensures that the residue is neutral when buried in the hydrophobic interior of the protein, consistent with expectations based on the energetic cost of removing a charged moiety from water. The shifts in pKa increase the pH sensitivity of thermodynamic stability. Using staphylococcal nuclease as a test system we engineered many variants with pairs of ionizable groups (Lys or Glu with anomalous pKa values) and demonstrated that this was sufficient to convert this protein into a pH switch that unfolds cooperatively near pH 7. Constructs with pairs of buried Lys residues are folded at high pH and unfolded by decreasing pH and constructs with pairs of buried Glu or Asp are folded at low pH and unfolded by increasing pH. Systematic engineering of several switch proteins with ionizable groups buried in different locations demonstrated that this approach is robust. Based on general thermodynamic principles, this strategy should be transferable to other proteins.