An unfilled and unstabilized peroxide-cured nitrile rubber containing 33 wt% acrylonitrile units was aged at two temperatures (100 and 120°C) under aerobic and anaerobic conditions up to 21 days. The bulk properties were analyzed by performing the following measurements: hardness, compound density, volatile content, mechanical (tensile strength, elongation at break), dynamic mechanical behavior including the glass transition temperature (Tg), and crosslink density by equilibrium solvent swell and time-domain Nuclear Magnetic Resonance (NMR) by the Double Quantum (DQ) technique. It was found that highest property changes were achieved with the 120°C aerobically aged samples compared to anaerobic conditions. At 100°C, the aerobic and anaerobically aged samples were generally similar in property change, likely due to the short aging times employed. All aged samples display a trend of increasing crosslink density (solvent swell and NMR), hardness, and Tg. Regularization of the DQ NMR results revealed the presence of high and low crosslink zones within the rubber matrix which shift and broaden upon heat aging due to a combination of chain breaking and crosslink building reactions. This investigation has shed new light on the prevalence of these reactions in thermally aged NBR with respect to bulk property changes. In addition, it is shown that the knowledge of the distribution of the crosslinks is invaluable to comprehend bulk properties.