A comparative study of the barrier properties of cast film of polypropylene carbonate (PPC) and cast film of poly(lactic acid) (PLA) has been made in this paper. Dynamic transmission measurements were conducted to obtain the barrier properties for oxygen transmission and for water vapour transmission. A special algorithm fminsearch in Matlab was used to adapt an exponential expression to the measured values. In this way the time needed to reach a 95% level of steady state was made possible to identify. The oxygen permeability was lower for PPC compared to PLA and this could be explained by a much higher positive enthalpy of solution for oxygen in PPC. The enthalpy of diffusion was close to similar for both PPC and PLA and was higher than for enthalpy of solution. The enthalpy of water vapour permeability was higher for PPC compared to PLA and this resulted in substantially higher water vapour permeability for PLA. The uptake of water vapour was also higher for PLA compared to PPC as shown by dynamic vapour sorption measurements. Differential scanning calorimetry confirmed that PPC is an amorphous polymer and that the PLA used in this study had a minimum of crystallinity which made it comparable to PPC.