The interaction of kaempferol (kaemp), a natural flavonoid to which antioxidative, anti-inflammatory and cardio-protective biological activities have been attributed, with human serum albumin (HSA), the main in vivo transporter of exogenous substances, was investigated by steady-state, synchronous fluorescence and circular dichroism spectroscopies. The binding constant, K, and number of binding sites, n, were computed using literature models that showed satisfactory agreement and revealed a strong interaction (K approximately 3.5x10(5)M(-1), n approximately 1). The binding process was investigated at temperatures in the range 298-313K, allowing for the evaluation of the thermodynamic parameters, which indicate the occurrence of hydrogen bonding interactions. The distance between kaemp and the tryptophan residue of HSA was estimated at 2.7nm using Förster's theory of nonradiative resonance energy transfer. Using circular dichroism we evidenced some degree of HSA defolding upon binding.