This paper demonstrates how the cell morphology of extruded expanded poly-propylene (EPP) may be related to the thermal decomposition behavior of chemical blowing agents (CBAs) used in their production. The thermal analysis techniques used in this study were differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Tests were completed on six of the CBAs now available for use with PP. It was found that agents with a higher decomposition temperature and rate of gas evolution generally produced foam samples with a smaller cell size and higher cell density. This information will be beneficial to producers and users of CBAs.