The interactions of dichlorprop (DCP) and diquat dibromide (DQ) herbicides with human serum albumin (HSA) protein were studied by UV absorption, fluorescence, synchronous fluorescence and circular dichroism (CD) spectroscopy. Both DCP and DQ quenched the fluorescence emission spectrum of HSA through the static quenching mechanism. The Stern-Volmer quenching constant, binding constant, the number of binding sites and thermodynamic parameters were determined at 288 K, 298 K, 310K and 318 K. In HSA-DCP and HSA-DQsystems, an increase in temperature led to a decrease in the Stern-Volmer quenching constant and binding constant. One binding site was obtained for DCP and DQ on HSA. It was found that DCP can bind to HSA with higher affinity than DQ, Negative Delta H and positive Delta S values were obtained for the binding processes between protein and herbicide molecules. This result displayed that electrostatic interactions play a major role in the formation of HSA-DCP and HSA-DQ complexes. The binding processes were exothermic reactions and spontaneous. In addition, synchronous fluorescence and CD spectra of HSA revealed that the binding of DCP to HSA did not cause a significant conformational change in protein, but the interaction of DQ with HSA led to an alteration in the protein structure. (C) 2014 Elsevier B.V. All rights reserved.