The hydrogenation of NBR in an aromatic solvent with tris-(triphenylphosphane)-rhodium-(I) halide as catalyst under hydrogen pressures of from 20 to 350 bars and at temperatures in the range from 100.degree. to 145.degree. C. leads to products having degrees of hydrogenation of at least 90% and outstanding properties when the catalyst concentration amounts to between 0.05 and 0.6% by weight, based on solid rubber, and when hydrogenation is carried out in the presence of at most 2% by weight of triphenylphosphane.