Hydrogenated nitrile butadiene rubbers (HNBRs) are hydrogenated copolymers of acrylonitrile and butadiene, which are often used in composites or polymer blends. These copolymers are usually cured with peroxides or vulcanized with sulfur to reinforce their mechanical resistance and improve their chemical stability. However, using such crosslinking reagents can be problematic for high value systems where residual H2O2 or S can be detrimental for the application. To address this limitation, we studied the thermally induced crosslinking of HNBR at high temperature (240 degrees C) with oxygen. To understand the impact of conditions (temperature, time, and atmosphere) on the chemical structure and the mechanical properties of HNBR, different thermal treatments were investigated. We show that HNBR forms a ladder structure during treatment in the presence of O-2 which result in a reinforcement of the elastomer. Tensile tests and DSC show both alkene and nitrile moieties are involved in the reaction, leading to a mechanical resistance comparable with a HNBR crosslinked with peroxides or sulfur. These findings will help achieving a better control on the crosslinking to provide HNBRs with desired properties. (C) 2018 Wiley Periodicals, Inc.