The diffusion coefficient is an important parameter in the field of migration and diffusion. Different diffusion theories have been established to estimate the diffusion coefficient, including the weight gain rate. However, the research was based on the migration of different solvents into simple polymers. Few reports focused on the migration of components from propellant and liner into ethylene-propylene terpolymer (EPDM) insulation. In this paper, a new parameter named surface area diffusion rate was proposed to measure the migration of components from propellant and liner at interfaces of EPDM insulation, and the comparison with the traditional weight gain rate was made. The result shows that the surface area diffusion rate is independent of the shape of the material and is more reliable than previously used methods. Furthermore, the relationship of the surface area diffusion rate with time and temperature was investigated. In addition, the diffusion activation energy and the relationship of surface area diffusion rate with the bonding properties were discussed.