Degradation behavior of poly(2,3-dihydrofuran) (PDHF) was investigated. PDHF was prepared by cationic polymerization of 2,3-dihydrofuran (DHF) with boron trifluoride (BF3) in toluene at similar to 78 degrees C. The rate of the degradation depended upon the relative surface area of PDHF samples. The larger the surface area of the PDHF sample, the faster the molecular weight decreased. An oxidative degradation mechanism was confirmed, because lactone and acetal structures were detected in the degraded PDHF samples by H-1, C-13 NMR, IR, and ESR spectra. No degradation proceeded under a nitrogen atmosphere.