A comparative study of the thermostability of NAD+-dependent formate dehydrogenases (FDHs; EC 126.96.36.199) from both methylotrophic bacteria Pseudomonas sp. 101 and Moraxella sp. C1, the methane-utilizing yeast Candida boidinii, and plants Arabidopsis thaliana and Glycine max (soybean) was performed. All the enzymes studied were produced by expression in E. coli cells. The enzymes were irreversibly inactivated in one stage according to first-order reaction kinetics. The FDH from Pseudomonas sp. 101 appeared as the most thermostable enzyme; its counterpart from Glycine max exhibited the lowest stability. The enzymes from Moraxella sp. C1, C. boidinii, and Arabidopsis thaliana showed similar thermostability profiles. The temperature dependence of the inactivation rate constant of A. thaliana FDH was studied. The data of differential scanning calorimetry was complied with the experimental results on the inactivation kinetics of these enzymes. Values of the melting heat were determined for all the enzymes studied.