Using a newly developed high-pressure autoclave, which has two sapphire windows on the walls, we visually observed the batch physical foaming of polymer-clay nanocomposites to understand the effect of nano-sized clay on the initial stage of foaming. With CO2 as a physical foaming agent, polypropylene-montmorillonite clay nanocomposites were foamed at 150degreesC. A high-speed digital camera with a microscope could observe the bubble nucleation and bubble growth behavior of the early stage of foaming in situ. The series of micrographs was analyzed in order to investigate the effect of clay content on bubble nucleation and growth. The experiments, together with CO(2-)solubility and diffusivity data, show that the clay enhances bubble nucleation as a nucleation agent and retards the growth of bubbles at the early stage of foaming.(C) 2004 Society of Plastics Engineers.