We report hydrogen deuterium exchange by mass spectrometry (HDX-MS) as a function of temperature in a thermophilic dihydrofolate reductase from Bacillus stearothermophilus (Bs-DHFR). Protein stability, probed with circular dichroism, established an accessible temperature range of 10 degrees C to 55 degrees C for the interrogation of HDX-MS. Although both the rate and extent of HDX are sensitive to temperature, the majority of peptides showed rapid kinetics of exchange, allowing us to focus on plateau values for the maximal extent of exchange at each temperature. Arrhenius plots of the ratio of hydrogens exchanged at 5 h normalized to the number of exchangeable hydrogens vs. 1/T provides an estimate for the apparent enthalpic change of local unfolding, DeltaH degrees (unf(avg)). Most regions in the enzyme show DeltaH degrees (unf(avg)) < or = 2.0 kcal/mol, close to the value of kT; by contrast, significantly elevated values for DeltaH degrees (unf(avg)) are observed in regions within the core of protein that contain the cofactor and substrate-binding sites. Our technique introduces a new strategy for probing the temperature dependence of local protein unfolding within native proteins. These findings are discussed in the context of the demonstrated role for nuclear tunneling in hydride transfer from NADPH to dihydrofolate, and relate the observed enthalpic changes to two classes of motion, preorganization and reorganization, that have been proposed to control the efficiency of hydrogenic wave function overlap. Our findings suggest that the enthalpic contribution to the heavy atom environmental reorganizations controlling the hydrogenic wave function overlap will be dominated by regions of the protein proximal to the bound cofactor and substrate.