Herein we report the interaction of 3-hydroxyflavone (3HF) with various isomeric forms of Human Serum Albumin (HSA), namely, the N-isoform (or native HSA at pH 7.4) and the B-isoform (at pH 9.2). Spectroscopic signatures of 3HF reveal that the interaction of 3HF with the N-isoform of HSA results in significant lowering of absorbance of the neutral species (lambda(abs) similar to 345 nm) with concomitant increase of the anionic species (lambda(abs) similar to 416 nm) whereas interaction with the B-isoform of HSA leads to selective enhancement of absorbance of the anionic species. The fluorescence profile of 3HF displays marked increase of intensity of the proton transferred tautomer (lambda(em) similar to 538 nm) as well as the anionic species (lambda(em) similar to 501 nm) for both the forms of the protein. However, analyses of the associated thermodynamics through temperature-dependent isothermal titration calorimetric (ITC) indicate that the interaction of 3HF with the N-isoform of HSA is more enthalpic in the lower temperature limit while the entropy contribution predominates in the higher temperature limit. Consequently, the 3HF-HSA (N-isoform at pH 7.4) interaction reveals an unusual thermodynamic signature of a positive heat capacity change (Delta C-p = 3.84 kJ mol(-1)K(-1)) suggesting the instrumental role of hydrophobic hydration. On the contrary, the 3HF-HSA (B-isoform at pH 9.2) interaction shows qualitatively reverse effect. Consequently, the interaction is found to be characterized by an enthalpy-dominated hydrophobic effect (negative heat capacity change, Delta C-p = -1.15 kJ mol(-1)K(-1)) which is rationalized on the basis of the nonclassical hydrophobic effect.