Poly(ethylene oxide) (PEO) modified cytochrome c cross-linked with glutaraldehyde (PEO-GA-cyt.c) kept redox activity in PEO oligomers for about 1 year. The peak current for PEO-GA-cyt.c fixed on the ITO glass electrode by the electrochemical adsorption gradually decreased and no redox response was observed about a half year later. A CD spectrum for PEO-GA-cyt.c in PEO200 (average molecular weight of 200, containing 0.5 mol/l KCl) suggested that a large conformational change did not occur for about 1 year. This improved stability of PEO-GA-cyt.c was considered attributed to both the cross-linking with CA and the use of PEO as a solvent.