A series of PMR-15 resin specimens were isothermally aged at 288, 316, and 343 degrees C over a range of time. For PMR-15 aged at 288 degrees C, the samples were also subjected to different aging environments including: ambient air, dry air, inert (argon), and pressurized air (0.414 MPa). Nanoindentation was performed to characterize localized mechanical properties as well as the development and growth of the oxidative layer. The measured increase in stiffness in the specimen surface oxidation layer is a manifestation of the chemical changes in the polymer occurring during oxidation. The average elastic modulus in the oxidized region is relatively insensitive to variations in aging temperature, time, and the environments. The thickness of the oxidative layer is observed to increase in the early stages of oxidation and the oxidation process eventually approaches an auto-retardation state. Aging under elevated pressure increases the thickness growth rate of the oxidation layer, while there is no significant difference in growth rate for specimens aged in dry air versus those aged in ambient air. It is shown that the measured average thickness of the oxidation layer and the transition region determined by the nanoindenter is in good agreement with optical microscopy measurements for all conditions considered.