The cross-link density measurements by an MQ pulse sequence on a low-field spectrometer highlighted an increase in the cross-linked density of SBR up to 504 h by 33 %, which remained unchanged up to 988 h. In contrast, NR had the same cross-link density up to 168 h, which then dropped by 10 % approaching 504 h. The stagnation in cross-link density up to 168 h was attributed to a modification of polysulfidic bonds to the lower; disulfidic and monosulfidic variants.