AbstractSulfur‐crosslinked nitrile butadiene rubber (s‐NBR) was found to be devulcanized when it was heated with nitrobenzene at 200°C for 3 h. The tetrahydrofuran (THF)‐soluble fraction from s‐NBR heated with nitrobenzene was purified by reprecipitation with THF/n‐hexane, chloroform/n‐hexane, and THF/n‐hexane systems and was then characterized by means of Fourier transform infrared (FTIR) spectroscopy, 1H‐NMR, gel permeation chromatography, dynamic thermogravimetry/differential thermal analysis (DTA), and differential scanning calorimetry (DSC). FTIR and 1H‐NMR results revealed that the THF‐soluble fraction contained aromatic rings derived from nitrobenzene. Furthermore, the molecular weight of the THF‐soluble fraction was much lower than that of the parent noncrosslinked poly(acrylonitrile‐co‐butadiene). Although the weight loss of THF‐soluble fraction began at a lower temperature than that of the nonheated original nitrile butadiene rubber, the residual weight at 700°C tended to be higher for the former. This tendency became more marked with increasing time of heat treatment with nitrobenzene. The DSC‐determined glass‐transition temperature of the THF‐soluble fraction was higher than that of the original s‐NBR. To elucidate the devulcanization mechanism, we investigated two types of model reactions; one was the reaction of diphenyl disulfide with nitrobenzene, and the other was the reaction of polybutadiene with nitrobenzene. The former reaction, carried out at 250°C in diphenyl ether, yielded diphenyl sulfide with a loss of diphenyl disulfide and nitrobenzene. The use of a higher molar ratio of nitrobenzene to diphenyl disulfide resulted in a depression of diphenyl sulfide formation. The reaction of p‐chloronitrobenzene with diphenyl disulfide also gave diphenyl sulfide. The reaction of polybutadiene with nitrobenzene at 200°C resulted in the backbone scission of the polymer. The THF‐soluble solid product of the latter model reaction was found by FTIR and 1H‐NMR to contain aromatic rings derived from nitrobenzene. The devulcanization mechanism is discussed on the basis of a comparison of the results of the model reactions with those of the s‐NBR devulcanization. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3342–3353, 2004