Poly(ethylene oxide) (PEO) oligomers were used as a solvent for electrochemical responses of heme-proteins. Heme-proteins such as hemoglobin and myoglobin were chemically modified with PEO chains to improve compatibility with the solvent PEG. Such PEG-modified heme-proteins show quasireversible electrochemical redox reactions in the PEO containing supporting electrolytes. In PEO solution, the electron transfer reaction between electrode and the proteins was clearly detected in both the homogeneously dissolved system and the cast system onto the electrode. The redox reaction was detected in a wide temperature range up to 140 degrees C. Solvent PEO with greater molecular weight shows better thermal stability of the heme-proteins. This excellent thermal stability of the heme-proteins was concluded to be due to the characteristics of solvent PEG, ie PEO has much less molecular motion than water molecules even at higher temperature. PEO was concluded to be an excellent solvent for electrochemical studies and application of redox active proteins in a wide temperature range.