Ionomers have been studied in detail and have gained widespread commercial use. Interested in altering the ratio of CO2/O-2 permeability (i.e., permselectivity) of films for use in packaging respiring produce, we pressed sodium-neutralized poly(ethylene methacrylic acid) ionomers into films at 120-160degreesC and investigated their thermal, morphological, and permeability properties. The heat treatment of the ionomers at 160 C increased the gas permeability for O-2 more than for CO2 and reduced the CO2/O-2 permselectivity ratio from 4.1 to 1.6. The reasons for these changes in the permeability characteristics of the ionomers are not understood but could be related to a reduction in the polyethylene (PE) crystallinity and the destruction of the aggregates into smaller, dispersed clusters. These results were supported by differential scanning calorimetry data and scanning electron micrographs. This suggests that the heat treatment of ethylene-methacyrlic ionomers may improve their potential for applications as selective barriers for modified-atmosphere packaging of respiring produce.