Hydroxyl terminated polybutadiene resin (HTPB) based solid propellants are being currently used in solid rocket motors of launch vehicles. HTPB-XT (HTPB blended with the hydrocarbon oil, Extol-21) was developed to enhance the pot life of the propellant slurry and to overcome the problems in the propellant processing due to the shorter pot life of present HTPB propellants. The diffusion study of labile chemical species in HTPB and HTPB-XT propellants (with low and high viscosity hydrocarbon oils) was carried out. Analog cartons simulating the propellant of the main motor were cast with 120 kg mix level propellant and subjected to an aging study at room temperature and an accelerated condition at different intervals over a period of 100 days. Samples from the cartons were subjected to Soxhlet extraction and analyzed for concentration of chemical species in insulation rubber as well as in propellant. The change in concentration of species from the insulation to interior propellant was also studied and compared with HTPB propellant. The diffusion of the other chemicals along with plasticizer was also observed in these studies. Nonax-D, a solid ingredient also migrated due to its solubility in the propellant matrix but not due to the movement of solid particles. Due to the migration of propellant chemicals into insulation, there was a significant reduction in their initial concentrations in the propellant regions adjacent to the insulation surface. The rate of diffusion of the chemically unbound species was affected by temperature and time. The results of viscosity build-up, mechanical, interfacial, ballistic properties for HTPB and HTPB-XT propellants are presented and discussed.