AbstractThin films of a polyester of lactic and glycolic acid were prepared to give controlled amounts of disk spherulites. The spherulite contents ranged from zero to 100% and were accurately measured. The stress‐strain properties of the films were then determined at 60°C, i.e., about 20°C above the glass transition temperatureTg. The mechanical behavior varied quite systematically with spherulite content and displayed little dependence on spherulite size. It was found that much of the mechanical data could be reasonably well described by a simple composite model. In addition, the yield strain as well as the strain to break could be principally coupled to the deformation of only the amorphous phase. SEM and optical microscopy studies supported the above conclusion, also demonstrating that the isolated spherulites adhered well to the amorphous matrix and behaved as stress concentrators in the system when the deformation temperature was abov