The co-chaperonin protein 10 (cpn10) is a heptameric ring-shaped protein present in most organisms. It works in conjunction with the chaperonin protein 60 (cpn60) in an ATP-dependent process to assist folding a range of substrate polypeptides. Cpn10 from the hyper-thermophilic bacterium Aquifex aeolicus (Aacpn10) contains a 25-residue C-terminal extension in each monomer, not found in any other cpn10 protein. Both Aacpn10 and a mutant where the tail has been removed (Aacpn10del-25) adopt heptameric structures with similar thermal and chemical stabilities. In addition, the equilibrium and kinetic unfolding/dissociation and refolding/reassembly reactions are not affected by the presence or absence of the tail. The presence of the tail, however, increases the affinity between the subunits in the heptamer and limits the formation of ordered aggregates of the heptamers at high temperatures and high protein concentrations. Comparative studies on mesostable cpn10 heptamers from human mitochondria (hmcpn10) and Escherichia coli (GroES) reveal that the extreme stability of Aacpn10 originates from increased stability of individual monomers. The stability profile (i.e., the correlation between free energy and temperature) for Aacpn10 is shifted upwards (i.e., higher stability at each temperature) and to the right (i.e., maximum stability at higher temperature) as compared to that of GroES. This is the first thermodynamic analysis of how hyper-thermostability is achieved in an oligomeric protein system.