The dynamic evolution of gaseous hydrogen, methane, and carbon dioxide in the gamma- and He-4-ion radiolyses of solid polymers was investigated. The polymers used include low-density and high-density polyethylene, polypropylene, polystyrene, poly (methyl methacrylate), Nylon 11, Nylon 6, and poly(dimer acid-co-alkyl polyamine). An inline quadrupole mass spectrometer was utilized to monitor the dynamic profiles of the gases produced in the radiolysis. One- and two-dimensional numerical diffusion models were developed to simulate and extract optimum diffusion coefficients and gas yields from the experimental dynamic gas profiles. It was found that the dynamic evolution of molecular hydrogen from the bulk polymer is controlled by its diffusion in most cases, such as CO2 in poly(methyl methacrylate). In the gamma radiolysis of some polymers such as low-density polyethylene and polypropylene, the dynamic evolution of methane is only partially controlled by the diffusion process, and some other postirradiation process is a factor. It is concluded that the simulation method developed in this article is helpful in understanding and predicting the mechanisms of gas evolution in the radiolysis of solid polymers.