The need for the development of polymeric materials based on renewable resources has led to the development of poly(lactic acid)(PLA)which is being produced from a feedstock of corn rather than petroleum. The present study examines the permeation of nitrogen, oxygen, carbon dioxide, and methane in amorphous films of PLA cast from solution. The properties of PLA are compared to other commodity plastics and it is shown that PLA permeation closely resembles that of polystyrene. At 30°C, N2 permeation in PLA is 1.3 (10-10 cm3 (STP)cm/cm2 s cmHg)and the activation energy is 11.2 kJ/mol. For oxygen the corresponding values are 3.3 (10-10 cm3(STP)cm/cm2 s cm Hg)and 11.1 kJ/mol. The values for carbon dioxide permeation are 1.2 (10-10 cm3 (STP)cm/cm2 s cmHg)and 6.1 kJ/mol. For methane values of 1.0 (10-10 cm3 (STP)cm/cm2 s cmHg)and an activation energy of 13.0 kJ/mol are found. Studies with pure gases show that polymer chain branching and small changes in : stereochemical content have no effect on permeation properties. Crystallinity is found to dominate permeation properties in a biaxially oriented film. The separation factor for a CO2/CH4 mixed gas system is measured between 0 and 50°C and does not deviate significantly from the calculated ideal separation factor; at 0°C the separation factor is 16, a value that suggests continued studies of PLA as a separation medium are warranted.