Revival studies of Aeropyrum pernix show that the viability of cells and cell recovery after heat treatment depends on the temperature of treatment. Differential scanning calorimetry (DSC) is used to analyze the relative thermal stabilities of cellular components of A. pernix and to identify the cellular components responsible for the observed lag phase and reduced maximum growth following a heat treatment. DSC thermograms show 5 visible endothermic transitions with 2 major transitions. DSC analysis of isolated crude ribosomes aids the assignment of the 2 major peaks observed in whole-cell thermograms to denaturation of ribosomal structures. A comparison of partial and immediate full rescan thermograms of A. pernix whole cells indicates that both major peaks represent irreversible thermal transitions. A DNA peak is also identified in the whole-cell thermogram by comparison with the optical data of isolated pure DNA. DNA melting is shown to be irreversible in dilute solution, whereas it is partially reversible in whole cells, owing at least in part, to restricted volume effects. In contrast to mesophilic organisms, hyperthermophilic A. pernix ribosomes are more thermally stable than DNA, but in both organisms, irreversible changes leading to cell death occur owing to ribosomal denaturation.