Process for reducing the molecular weight of nitrile rubber, characterized in that the nitrile rubber is contacted, optionally in the presence of a co-olefin, and reacted with a ruthenium complex catalyst of the general formula where X 1 and X 2 represent identical or different anionic ligands, preferably halogen, more preferably F, Cl, Br, I and especially preferably Cl, R 1 represents a linear or branched, aliphatic C 1 -C 20 -alkyl, C 3 -C 20 -cycloalkyl, C 5 -C 20 -aryl, CHCH 3 -CO-CH 3, preferably methyl, ethyl, isopropyl, isoamyl, t-butyl, CHCH 3 -CO-CH 3, cyclohexyl or phenyl, R 2 represents hydrogen, halogen, C 1 -C 6 -alkyl or C 1 -C 6 -alkenyl, R 3 an electron-withdrawing radical, preferably -F, -Cl, -Br, -I, -NO, -NO 2, -CF 3, -OCF 3, -CN, -SCN, -NCO, -CNO, -COCH 3, -COCF 3, -CO-C 2 F 5, -SO 3, -SO 2 - N(CH 3) 2, arylsulfonyl, arylsulfinyl, arylcarbonyl, alkylcarbonyl, aryloxycarbonyl, or -NR 4 -CO-R 5 where R 4 and R 5 are the same or different and may each independently be H, C 1 -C 6 -alkyl, perhalogenated C 1 -C 6 -alkyl, aldehyde, ketone, ester, amide, nitrile, optionally substituted aryl, pyridinium-alkyl, pyridinium-perhaloalkyl or optionally substituted C 5 -C 6 cyclohexyl, a C n H 2n Y or C n F 2n Y radical with n = 1 to 6 and Y an ionic group or a radical of one of the formulae (EWG 1), (EWG 2) or (EWG 3) where R 6, R 7, R 8, R 9, R 10, R 11 independently represent H, C 1 -C 6 -alkyl, C 1 -C 6 -perhaloalkyl or C 5 -C 6 -aryl and R 9, R 10, R 11 may form a heterocycle, X 3 represents an anion, halogen, tetrafluoroborate [BF 4] -), [tetrakis-(3,5-bis(trifluoromethyl)phenyl)borate]([BARF] -), hexafluorophosphate ([PF 6] -), hexafluoroantimonate ([SbF 6] -), hexafluoroarsenate ([AsF 6] -) or trifluoromethylsulfonate ([(CF 3) 2 N] -); R 4 and R 5 together with the N and the C to which they are bonded may form a heterocycle of the formula (EWG 4) or (EWG 5) where hal is halogen and R 12 is hydrogen, C 1 -C 6 -alkyl, C 5 -C 6 -cycloalkyl or C 5 -C 6 -cycloaryl, L represents an NHC ligand of the general formula or where R 13 is hydrogen, C 1 -C 6 -alkyl, C 3 -C 30 -cycloalkyl or C 5 -C 30 -aryl, R 14 and R 15 are the same or different and are linear or branched C 3 -C 30 -alkyl, C 3 -C 30 -cycloalkyl, C 5 -C 30 -aryl, C 5 -C 30 -alkaryl, C 5 -C 30 -aralkyl, with optionally up to 3 heteroatoms, preferably isopropyl, i-butyl, tert-butyl, cyclohexyl or phenyl. X 1 and X 2 represent identical or different anionic ligands, preferably halogen, more preferably F, Cl, Br, I and especially preferably Cl, R 1 represents a linear or branched, aliphatic C 1 -C 20 -alkyl, C 3 -C 20 -cycloalkyl, C 5 -C 20 -aryl, CHCH 3 -CO-CH 3, preferably methyl, ethyl, isopropyl, isoamyl, t-butyl, CHCH 3 -CO-CH 3, cyclohexyl or phenyl, R 2 represents hydrogen, halogen, C 1 -C 6 -alkyl or C 1 -C 6 -alkenyl, R 3 an electron-withdrawing radical, preferably -F, -Cl, -Br, -I, -NO, -NO 2, -CF 3, -OCF 3, -CN, -SCN, -NCO, -CNO, -COCH 3, -COCF 3, -CO-C 2 F 5, -SO 3, -SO 2 - N(CH 3) 2, arylsulfonyl, arylsulfinyl, arylcarbonyl, alkylcarbonyl, aryloxycarbonyl, or -NR 4 -CO-R 5 where R 4 and R 5 are the same or different and may each independently be H, C 1 -C 6 -alkyl, perhalogenated C 1 -C 6 -alkyl, aldehyde, ketone, ester, amide, nitrile, optionally substituted aryl, pyridinium-alkyl, pyridinium-perhaloalkyl or optionally substituted C 5 -C 6 cyclohexyl, a C n H 2n Y or C n F 2n Y radical with n = 1 to 6 and Y an ionic group or a radical of one of the formulae (EWG 1), (EWG 2) or (EWG 3) R 6, R 7, R 8, R 9, R 10, R 11 independently represent H, C 1 -C 6 -alkyl, C 1 -C 6 -perhaloalkyl or C 5 -C 6 -aryl and R 9, R 10, R 11 may form a heterocycle, X 3 represents an anion, halogen, tetrafluoroborate [BF 4] -), [tetrakis-(3,5-bis(trifluoromethyl)phenyl)borate]([BARF] -), hexafluorophosphate ([PF 6] -), hexafluoroantimonate ([SbF 6] -), hexafluoroarsenate ([AsF 6] -) or trifluoromethylsulfonate ([(CF 3) 2 N] -); R 4 and R 5 together with the N and the C to which they are bonded may form a heterocycle of the formula (EWG 4) or (EWG 5) R 12 is hydrogen, C 1 -C 6 -alkyl, C 5 -C 6 -cycloalkyl or C 5 -C 6 -cycloaryl, L represents an NHC ligand of the general formula or R 13 is hydrogen, C 1 -C 6 -alkyl, C 3 -C 30 -cycloalkyl or C 5 -C 30 -aryl, R 14 and R 15 are the same or different and are linear or branched C 3 -C 30 -alkyl, C 3 -C 30 -cycloalkyl, C 5 -C 30 -aryl, C 5 -C 30 -alkaryl, C 5 -C 30 -aralkyl, with optionally up to 3 heteroatoms, preferably isopropyl, i-butyl, tert-butyl, cyclohexyl or phenyl.