Ion conductive polyether oligomers were used as a matrix for the electrochemical redox reaction of heme proteins. The redox activity and stability of poly(ethylene oxide)modified myoglobin (PEG-Mb) in PEO oligomers were investigated at various temperatures with visible spectroscopy. Synthesized PEG-Mb was soluble in PEO oligomers with average molecular weight of 200 (PEO200) at room temperature, and it exhibited visible spectra for the oxidized and reduced form at 120 degrees C which is similar to those at 30 degrees C. PEO-Mb in PEO200 was reduced and oxidized repeatedly at 80 degrees C for at least 2.5 h by changing the applied potential. The effect of water content on the thermal stability of PEG-Mb in PEO200 was also demonstrated at 80 degrees C. The absorbance of the oxidized PEO-Mb in PEO200 at 80 degrees C was stable for at least 6 h when the water content was less than 10 wt%. The PEO oligomers, containing water of 13.7 wt% and more, made the PEG-Mb unstable at 80 degrees C. The thermal stability of PEO-Mb in PEO oligomers was considerably reduced by the addition of water for more than 10 wt%. The origin of this extreme thermal stability of PEG-Mb was concluded to be due to the PEG oligomers used as a solvent.