Reversible thermal denaturation of 33 genetic variants of human serum albumin (HSA) appeared to be a two-state process when studied by circular dichroism (CD). Fourteen single-residue variants have T m values (midpoint of denaturation) higher than, and nine have T m values lower than, their endogenous, wild-type counterpart. Nine single-residue variants have DH v values (van't Hoff enthalpy) higher than, and 14 have DH v values lower than, normal albumin. All types of combinations of positive and negative DT m values and D(DH v ) values were found. Good linear correlations between mutation-induced changes of a-helical content and D(DH v ) values, but not DT m values, were found especially for the variants mutated in domains I and III. The effect of altered chain length and glycosylation on T m and DH v was also studied. For all variants, no clear relationship was found between the changes in the thermodynamic parameters and the type of substitution, changes in protein charge or hydrophobicity. However, the protein changes taking place in domain I have a rather uniform effect (almost all of the nine variants have positive DT m values and negative D(DH v ) values, i.e., they denature more easily than normal albumin but they do so at a higher temperature). The present results can be of both protein chemical relevance and of clinical interest, because they could be useful when designing stable, recombinant HSAs for clinical applications.