This paper is concerned with microstructure and kinetics relating to bubble growth within a pentane loaded molten polystyrene sample and the work is of relevance to the commercial foaming of polymers achieved by pressure release at the exit of an extruder. Optical experiments were carried out using a Multipass Rheometer to follow the time evolution for melting pentane loaded polystyrene beads. Two protocols were followed and one resulted in the formation of a single melt phase. With subsequent pressure release, the growth kinetics of bubbles were followed together with theological measurements. A modified Newtonian model incorporating gas diffusion was developed and compared with the experimental data. Good matching of data could be achieved using realistic values of parameters and the model was able to predict early stage growth conditions. The model, as presented, still has many simplifications, however it does form the basis for further refinement. (C) 2004 Elsevier Ltd. All rights reserved.