DMTA measurements revealed stiffening at long weathering durations in NR that softened almost entirely at higher temperatures. The stiffening was pronounced in SBR. This contributed to an additional softening transition which showed significant modulus even at 100 °C. The chemical origins of these were evaluated by FTIR, which revealed modification of the surface layer into a stiff skin due to oxidation processes aided by various weathering parameters. The regions below this skin layer oxidized to a lesser extent due to the barrier-like behavior of the skin. AFM nanoindentation experiments of SBR weathered up to 504 h measured at various depths confirmed the oxidation-induced stiffening that imparted an additional transition in DMTA experiments.