Catalyst comprising at least one amidine or guanidine group of the formula (Ia) or (Ib) which is bonded to a silicon atom of a siloxane radical wherein L represents a linear or branched, divalent hydrocarbon radical having 1 to 20 C atoms, which optionally contains one or more nitrogen atoms, R 1 and R 0 independently of each other each represent a hydrogen radical or an alkyl or cycloalkyl or aralkyl radical having 1 to 8 C atoms, R 2 represents a hydrogen radical or an alkyl, cycloalkyl or aralkyl radical having 1 to 18 C atoms, which optionally contains hetero atoms and which optionally contains terminal primary or secondary amino groups, R 3 represents -NHR 5 or a hydrogen radical or an alkyl, cycloalkyl or aralkyl radical having 1 to 12 C atoms, R 5 represents a hydrogen radical or an alkyl, cycloalkyl or aralkyl radical having 1 to 18 C atoms, which optionally contains hetero atoms, wherein R 1 together with R 2 can also represent an alkylene radical having 2 to 6 C atoms, R 0 together with R 2 can also represent an alkylene radical having 3 to 6 C atoms, which optionally contains hetero atoms, R 2 together with R 3 can also represent an alkylene radical having 3 to 6 C atoms and R 2 together with R 5 can also represent an alkylene radical having 2 to 12 C atoms, characterized in that the siloxane radical is a polysiloxane radical A 3 of the formula wherein n represents an integer in the range from 10 to 200, R 8 represents a monovalent hydrocarbon radical having 1 to 12 C atoms and Z represents an amidine or guanidine group of the formula (Ia) or (Ib) or an aminoalkyl group or a radical R 9, wherein R 9 represents a monovalent alkyl or alkoxy radical having 1 to 6 C atoms. L represents a linear or branched, divalent hydrocarbon radical having 1 to 20 C atoms, which optionally contains one or more nitrogen atoms, R 1 and R 0 independently of each other each represent a hydrogen radical or an alkyl or cycloalkyl or aralkyl radical having 1 to 8 C atoms, R 2 represents a hydrogen radical or an alkyl, cycloalkyl or aralkyl radical having 1 to 18 C atoms, which optionally contains hetero atoms and which optionally contains terminal primary or secondary amino groups, R 3 represents -NHR 5 or a hydrogen radical or an alkyl, cycloalkyl or aralkyl radical having 1 to 12 C atoms, R 5 represents a hydrogen radical or an alkyl, cycloalkyl or aralkyl radical having 1 to 18 C atoms, which optionally contains hetero atoms, wherein R 1 together with R 2 can also represent an alkylene radical having 2 to 6 C atoms, R 0 together with R 2 can also represent an alkylene radical having 3 to 6 C atoms, which optionally contains hetero atoms, R 2 together with R 3 can also represent an alkylene radical having 3 to 6 C atoms and R 2 together with R 5 can also represent an alkylene radical having 2 to 12 C atoms, characterized in that the siloxane radical is a polysiloxane radical A 3 of the formula wherein n represents an integer in the range from 10 to 200, R 8 represents a monovalent hydrocarbon radical having 1 to 12 C atoms and Z represents an amidine or guanidine group of the formula (Ia) or (Ib) or an aminoalkyl group or a radical R 9, wherein R 9 represents a monovalent alkyl or alkoxy radical having 1 to 6 C atoms. n represents an integer in the range from 10 to 200, R 8 represents a monovalent hydrocarbon radical having 1 to 12 C atoms and Z represents an amidine or guanidine group of the formula (Ia) or (Ib) or an aminoalkyl group or a radical R 9, wherein R 9 represents a monovalent alkyl or alkoxy radical having 1 to 6 C atoms.