The present work investigates the effects of HID isotopic sub stitution on the structural and thermodynamic stability of bovine serum albumin (BSA) in aqueous solution over the temperature range of 5-90 degrees C. Using far-ultraviolet circular dichroism, we have compared protein unfolding pathways in H(2)O and D(2)O. Our results show that BSA possesses similar conformations in H(2)O and D(2)O at temperatures below 50 degrees C but follows different unfolding pathways at higher temperatures. The presence of D(2)O retards the occurrence of irreversible thermal denaturation in BSA, as evidenced by a higher onset temperature of 58 degrees C, in contrast to 50 degrees C in H(2)O center dot D(2)O exhibits a protective effect on the domain structure during the early stages of domain denaturation. Following incubation at 90 degrees C over a period of minutes, D(2)O causes a rapid aggregation of BSA molecules. This behavior is not observed in H(2)O solutions. Meanwhile, H/D substitution does not influence the reversible structural transformation of the protein in a significant manner. Partly renatured BSA in H(2)O and D(2)O undergoes very similar reversible structural transformations during a second heating cycle.