<p>Optimizing the accelerator/sulphur (A/S) ratio is essential for achieving the desired properties in sulphur-cured hydrogenated acrylonitrile butadiene rubber (HNBR). This study systematically investigates the crosslinking efficiency of sulphur-cured HNBR with acrylonitrile (ACN) contents of 21% and 39% as a function of the A/S ratio (0.5-12), encompassing conventional (CV), semi-efficient (semi-EV), and efficient vulcanization (EV) systems. Through moving die rheometry and equilibrium swelling analysis, optimal A/S ratios of 1 and 4 were identified for the 21% and 39% ACN HNBR, respectively, maximizing crosslink density. Infrared (IR) spectroscopy confirmed changes in network structure, including the consumption of residual unsaturation, while X-ray photoelectron spectroscopy (XPS) studies revealed the prevalence of different sulphur linkages. The enhanced tensile performance of the 39% ACN HNBR is attributed to a combination of increased physical crosslinks and strain-induced crystallization. This study establishes quantitative relationships between the A/S ratio and crosslink characteristics, providing a basis for tailoring sulphur vulcanization of HNBR.</p> Graphical abstract <p>Effect of Accelerator to Sulphur ratio on mechanical properties of HNBR with low anf high ACN content.</p>