The thermodynamic stability and temperature induced structural changes of oxidized thioredoxin h from Chlamydomonas reinhardtii have been studied using differential scanning calorimetry (DSC), near- and far-UV circular dichroism (CD), and fluorescence spectroscopies. At neutral pH, the heat induced unfolding of thioredoxin h is irreversible. The irreversibly unfolded protein is unable to refold due to the formation of soluble high-order oligomers. In contrast, at acidic pH the heat induced unfolding of thioredoxin h is fully reversible and thus allows the thermodynamic stability of this protein to be characterized. Analysis of the heat induced unfolding at acidic pH using calorimetric and spectroscopic methods shows that the heat induced denaturation of thioredoxin h can be well approximated by a two-state transition. The unfolding of thioredoxin h is accompanied by a large heat capacity change [6.0 +/- 1.0 kJ/(mol.K)], suggesting that at low pH a cold denaturation should be observed at the above-freezing temperatures for this protein. All used methods (DSC, near-UV CD, far-UV CD, Trp fluorescence) do indeed show that thioredoxin h undergoes cold denaturation at pH <2.5. The cold denaturation of thioredoxin h cannot, however, be fitted to a two-state model of unfolding. Furthermore, according to the far-UV CD, thioredoxin h is fully unfolded at pH 2.0 and 0 degrees C, whereas the other three methods (near-UV CD, fluorescence, and DSC) indicate that under these conditions 20-30% of the protein molecules are still in the native state. Several alternative mechanisms explaining these results such as structural differences in the heat and cold denatured state ensembles and the two-domain structure of thioredoxin h are discussed.