The redox potentials E-0' of bovine cytochrome c adsorbed on an 11-mercapto-1-undecanol/Au self-assembled monolayer electrode were studied through direct electrochemistry as a function of the temperature in non-isothermal experiments carried out in the presence of different anions and changes of the ionic strength. The thermodynamic parameters for protein reduction (DeltaH(rc)(0') and DeltaS(rc)(0')) re were determined for adsorbed and solution cytochrome and the differences in E-0' discussed in terms of the enthalpic and entropic contributions. The adsorption process seems to remove the ability of perchlorate anion to bind to the protein surface, while a certain direct interaction is still retained in the case of chloride and phosphate. A moderate increase in E-0' of adsorbed cytochrome was measured at increasing ionic strength and discussed in the light of the opposite effect observed for solution protein. (C) 2003 Elsevier B.V. All rights reserved.