As a step toward the elucidation of the mechanistic pathways governing the known bioactivity of polyoxometalates (POMs), two representative molecules of this class of chemicals, the wheel-shaped [NaP(5)W(30)O(110)]14- (P(5)W(30)) and the Keggin-type anion [H(2)W(12)O(40)]6- (H(2)W(12)), are shown, by two independent techniques, to interact with the fatty-acid-free human serum albumin (HSA). The excited-state lifetime of the single tryptophan molecule of this protein is dramatically decreased by the binding. The quenching mechanism is found to constitute the first example of energy transfer between HSA and POMs. Such molecular recognition is believed to be a key step for subsequent evolution of the systems. Circular dichroism (CD) was used to assess the structural effects of POM binding on HSA and to confirm the interaction revealed by fluorescence studies. CD experiments showed that the two POMs have different effects on the secondary structure of the protein. Binding P(5)W(30) partially unfolds the protein whereas H(2)W(12) has no remarkable effect on the structure of the protein.