The thermodynamics of melting of bacteriophage T4 lysozyme and four of its mutants have been measured by van't Hoff methods. The effect of pH has been explored and utilized to obtain the dependence of the enthalpy on temperature as suggested by Privalov and co-workers. The enthalpy change is a steep linear function of temperature. delta Cp is large and constant within experimental error. Changes in delta Hu are as large as 30% for a single point mutation. Changes in enthalpy are largely compensated by changes in entropy. Changes in stability, as measured by the free energy of unfolding, are smaller than those of delta H, but are very large in a relative sense, since delta G is very much smaller than delta H. Origins of the destabilization caused by mutations are discussed.