The binding of long-chain imidazolium ionic liquid (IL), 1-tetradecyl-3-methylimidazolium bromide (C(14)mimBr) to bovine serum albumin (BSA) was investigated by fluorescence spectra and surface tension. Fluorescence spectra show that tryptophan (Trp) residues, one of the intrinsic fluorophores in BSA, are buried in a hydrophobic microrenvironment with the addition of C(14)mimBr, which induces the denaturation of BSA. Moreover, the fluorescence quenching mechanism was determined to be static quenching. The equilibrium constant (K) and the number of binding sites (n) were calculated based on the results of fluorescence measurement. The critical aggregation concentration (CAC) and critical micelle concentration (CMC) under different BSA concentrations at various temperatures were investigated based on the surface tension plots. Surface tension indicates that C(14)mimBr binds to BSA through electrostatic attraction at low C(14)mimBr concentrations (below CMC) and through hydrophobic interaction at high C(14)mimBr concentrations (above CIVIC). Additionally, the thermodynamic parameters of micelle formation were determined. This study provides an understanding of the binding of C(14)mimBr to BSA. (C) 2009 Elsevier Ltd. All rights reserved.