Self-diffusion and solubility coefficients of four gas molecules (O-2, N-2, CH4, and CO2) in poly(2,6-dimethyl-1,4-phenylene oxide) have been investigated by means of molecular simulation using the COMPASS molecular mechanics force field. Diffusion coefficients were obtained from molecular dynamics (NVT ensemble) using up to 2 ns simulation times. Solubility coefficients were obtained by means of the Widom particle insertion method. Simulation values for O-2, N-2, and CH4 agree reasonably well with published data. Agreement was less satisfactory for CO2. Possible explanations for the CO2 results are discussed on the basis of the partial immobilization model and considerations of simulation time and the size of the simulation box.