Europium salts were dissolved in poly(ethylene oxide) (PEG) oligomers. The europium ions were revealed to be electrochemically redox-active in PEO oligomers. Trivalent europium ions (EU(3+)) showed red fluorescence (lambda(em) = 614 nm) in PEO oligomers, and a new intensive blue fluorescence (lambda(em) = 450 nm) was obtained after reducing the Eu3+ into Eu2+ by giving negative potential (-0.7 V versus Ag). Reversible fluorescence switching was also carried out by changing the given potential alternately. Sufficient color change required about 20 min because of slow electrode reaction. The fluorescence intensity from europium ions was affected by the molecular weight of matrix PEG. The amount of terminal hydroxyl groups and local viscosity were revealed to affect the fluorescence as the vibrational quenching and diffusion coefficient, respectively. The PEO oligomer with average molecular weight of 600 gave the most intensive fluorescence.