Halophilic malate dehydrogenase is stable in a limited concentration range of MgCl2 or CaCl2. Thermal deactivation of the protein at low concentrations of these divalent salts is very different from that occurring at high concentrations. In low salt, stability always increases as the temperature is lowered. In high salt, stability shows bell-shaped behaviour as a function of temperature: increasing to a maximum at 4 degrees C, and subsequently decreasing as the temperature is lowered. This is in contrast to other salts, for which the deactivation behaviour depends on the salt type but not on its concentration. Cofactor addition or replacement of H2O by D2O modify only the deactivation at low MgCl2 or CaCl2 concentrations. A pH transition between pH 7 and pH 8, however, modified enzyme deactivation at both low and high MgCl2 or CaCl2 concentrations. The pH effect on stability was also observed in other salts. By comparing the effect of CaCl2, MgCl2, and NaCl, a strong correlation was found between the minimum salt concentration required for the stabilisation of halophilic malate dehydrogenase and the hydration of the cation.