The crosslinking of carboxylated nitrile rubber (XNBR) with a special preparation based on zinc oxide/zinc peroxide (Struktol ZP1014) is studied. It is shown that ionic species, by metallic salts formation, and covalent crosslinks. by zinc peroxide decomposition, are formed. The complete process can be empirically split in two contributions, with mathematical modeling rendering a satisfactory fit to experimental data. However, infrared and calorimetric analyses support the contribution of at least three mechanisms to the torque build-up: a very fast ionic species formation process by the initial zinc oxide present; covalent crosslinks and additional ionic species formation coming from zinc peroxide decomposition and contributing at all times from the beginning but decaying with time;, and a third, not yet determined, contribution that accounts for the torque increment at long times.