The effects of polymer molecular weight and temperature on the physical aging of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is examined. Gas permeability and refractive index were monitored during the aging process for PPO film samples at three aging temperatures below the glass transition temperature. Comparisons between the two samples of PPO that differ widely in molecular weight reveal an insignificant difference, which support the notion that above a critical molecular weight range there is little influence on aging rate. Increased temperature, over the limited range of 35-55 degrees C, results in higher aging rates for films made from both PPO materials. The rate of aging decreases strongly with increasing film thickness over the range examined, similar to 0.4-25 mu m.