Further analyses of the skin and core by NMR T2 studies suggested a continuous increase in the skin fractions in SBR up to about 7 % for 988 h of weathering. In NR, the stiff fraction stabilized between 504 h and 988 h at about 4 %. It was also indicated that the bulk of NR probably remained unchanged due to prolonged weathering, whereas in SBR a gradient of stiffness existed beneath the stiff skin due to continuous cross-linking.