Horse heart cytochrome c (cyt.c) was modified with poly(ethylene oxide) (PEO) to solubilize it in PEO oligomers. The electrochemical redox reaction of PEO-modified cyt.c (PEO-cyt.c) was analyzed with visible spectrometry in PEO(200) (average molecular weight of 200) containing 0.5 mol . l(-1) KCl as an electrolyte. Although the PEO-cyt.c showed redox reaction in PEO(200), the apparent rate constant of PEO-cyt.c was much smaller than that of PEO-modified hemoglobin and PEO-modified myoglobin in PEO(200). The degree of PEO-modification did not affect the reduction rate constant of PEO-cyt.c in PEO(200). The circular dichroism (CD) spectrometry showed no detectable conformational change of PEO-cyt.c in phosphate buffer. On the other hand, CD spectra of PEO-cyt.c in PEO(200) were different from those in phosphate buffer. This conformational change might be the reason for slow electrode reduction of PEO-cyt.c in PE0(200).