A new process is provided for preparing nitrile rubbers by free-radical polymerization in an organic solvent and in the presence of specific modifier substances. This polymerization may be followed by hydrogenation to give likewise new hydrogenated nitrile rubbers, the hydrogenation advantageously taking place likewise in organic solvent. The optionally hydrogenated nitrile rubbers obtained are notable for having fragments of the employed modifier substances in the main polymer chain and/or as end groups. They can be prepared with a wide diversity of molecular weights and polydispersity indices, especially with very low polydispersity indices.