The copolymerization of a highly fluorinated cyclic monomer, octafluorocyclopentene (OFCPE, M-1), with ethyl vinyl ether (EVE, M-2) was investigated with a radical initiator in bulk. Despite the poor homopolymerizability of each monomer, the copolymerization proceeded successfully, and the molecular weights of the copolymers reached up to more than 10,000. Incorporation of the OFCPE units into the copolymer led to an increase in the glass-transition point. The copolymer composition was determined from H-1 NMR spectra and elemental analysis data. The molar fraction of the OFCPE unit in the copolymer increased and approached but did not exceed 0.5. The monomer reactivity ratios were estimated by the Yamada-Itahashi-Otsu nonlinear least-squares procedure as r(1),OFCPE = -0.008 +/- 0.010 and r(2,EVE) = 0.192 +/- 0.015. The reactivi.ty ratios clearly suggest that the copolymerization proceeds alternatively in the case of an excessive feed of OFCPE.