The dependence of formal potential (E(0)’) on temperature for cytochrome c in phosphate buffer solution at various pH was investigated by the microband gold electrode modified with a new promoter 4,6-dimethyl-2-mercaptopyrimidine (DMMP). The temperature dependence of the E(0)’ showed a biphasic character in slightly alkaline solution with a turning point at ca. 45 degrees. However, the E(0)’ values decreased monotonically with the increasing temperature in acidic and neutral media. The thermodynamic parameters of the electron transfer reaction for cytochrome c at the chemically modified microband electrode were estimated. The E(0)’ and the changes of standard Gibb’s free energy (Delta G(0)), entropy (Delta S-0) and enthalpy (Delta H-0) for the reduction of ferricytochrome c at 25 degrees C and pH 7.0 were 0.272V (vs. NHE) -26.2 kJ mol(-1), -123.7 J mol(-1) K-1 and -63.1 kJ mol(-1), respectively. The effect of buffer component on the E(0)’ was also studied in binding (phosphate) and nonbinding (Tris/cacodylic acid) neutral buffer media. The results showed that the E(0)’ in Tris/cacodylic acid buffer solution shifted positively by about 10 mV in comparison with that in phosphate buffer solution.