Poly(ethylene oxide)-modified human hemoglobin (PEO-Hb) was cast on the indium tin oxide (ITO) glass electrode from aqueous solution, and the dried electrode was soaked into salt-containing PEO oligomers (MW:400, 600, 1000). Their redox reaction was investigated with both cyclic voltammetry and UV-vis spectrophotometry at wide temperature from 30 to 160 degrees C. The electron transfer reactions of PEO-Hb cast on the ITO glass electrode were clearly detected in PEO oligomer (MW:400) at temperatures from 30 to 140 degrees C with cyclic voltammetry. The extraordinary thermal stability of PEO-Hb on the ITO glass electrode was observed in only PEO oligomers as a solvent which was never seen in buffer solution. The thermal stability was improved by increasing the molecular weight of solvent PEG. The absorbance at Soret band for PEO-Hb was quite stable for 10 h at 80 degrees C in PEO1000 (MW:1000). PEO-Hb on the ITO glass electrode was also stable for 3 h at 120 degrees C, but denatured gradually at 140 degrees C in PEO1000.