Oxygen diffusion coefficients, D, have been measured as a function of temperature over the range 5-35 degreesC in films of poly(ethylene-co-norbornene). In the technique employed, the time evolution of oxygen sorption into the film was monitored using the phosphorescence of singlet oxygen as a spectroscopic probe. Many properties of these amorphous glassy materials depend significantly on the ethylene-to-norbornene ratio in the copolymer. For example, for samples in which the norbornene content in the copolymer was increased from 35 to 62%, the glass transition temperature of the material increased over the range 79-178 degreesC. Nevertheless, with these same changes in chemical composition, values of D(25 degreesC) varied only slightly over the range (2-6) x 10(-8) cm(2) s(-1), and values of the apparent diffusion activation energy, E-act, varied slightly over the range 35-40 kJ mol(-1). The values of D(25 degreesC) obtained correlate reasonably well with calculated values of the polymer free volume as well as with the average free volume cavity size determined for these samples using positron annihilation lifetime spectroscopy. These data are consistent with models of diffusion that are based on the dynamic redistribution of local voids of critical volume into which the diffusing molecule can move. This study thus provides the foundation for attempts to regulate oxygen transport in these technologically and industrially important materials.