The effect of crystallization on the foaming of poly(epsilon-caprolactone), a semi-crystalline biodegradable polymer, was studied using batch processing and CO, as the physical blowing agent (PBA). Previous work on the same system suggested that the formation of high cell population densities of small open cells was crystalline fraction dependent. In this work, ultrasonic sensors were used to investigate the effect of crystallization on the cell nucleation mechanism through the monitoring of ultrasonic velocity, attenuation and specific volume characteristics. Analysis of the ultrasonic data allowed to determine the crystallization temperature and thus determined the effect of crystallization on the pressure at which phase separation occurred (degassing pressure). Clearly, the presence of crystals leads to a 5-10-fold increase in the degassing pressure with respect to the amorphous system. In addition, it was shown that the talc content did not significantly influence the degassing pressure for a full amorphous system. However, talc particles can drastically affect the degassing conditions through their role of precursors for crystal formation. Different mechanisms are proposed as possible explanations for the enhancement of cell nucleation in such semi-crystalline polymer systems. Crown Copyright (c) 2006 Published by Elsevier Ltd. All rights reserved.