A transparent plastic in which gas was formed by gamma irradiation, was used to investigate the mechanisms of gas bubble formation and growth. Bubble formation and swelling were studied for several environments, both external and internal to the plastic-gas systems. During irradiation at 100°C, larger, but fewer bubbles form in the plastic than on lower temperature irradiation followed by 100°C annealing. The data indicate that inclusions increase bubble formation and internal surfaces promote bubble growth. An interpretation of these results, when applied to nuclear fuel swelling, indicates that swelling may be decreased by increasing fission rate and by decreasing the available sites for bubble nucleation and growth.