Europium nitrate was dissolved in poly(ethylene oxide) (PEO) and complexed with some ligands. Fluorescence intensity based on trivalent (red) and divalent (blue) europium ions was enhanced by the complexation with 2,2':6',2'-terpyridine and 2,2'-bibenzoxazoline, respectively. These complexed europium ions were electrochemically redox-active in supporting electrolyte-containing PEG. Their redox potential was - 237 and - 326 mV(vs. Ag) in PEG, respectively. The complex of oxidized europium ion/2,2':6',2'-terpyridine complex and reduced europium ion/2,2'-bibenzoxazoline complex showed red and blue fluorescence, respectively. Mixed solutions of these two complexes showed the emission color change between red and blue repeatedly as a result of redox reaction.