Process for preparing a polyurethane comprising reacting at least one polyisocyanate with a polyol composition comprising at least one polyol in the presence of a catalyst composition comprising at least one compound selected from the group consisting of compounds of the general formula (Ia) or (Ib) wherein - n is 0 or 1 when X is a carbon atom, n is 2 when X is a sulfur atom, - n' is 0 or 1, - m is an integer from 1 to 6, - R is selected from monovalent, or divalent, or trivalent or polyvalent residues, wherein - when R is monovalent, R is H, OH, substituted or unsubstituted, linear or branched C 1 -C 18 alkyl, C 5 -C 7 cycloalkyl which can be substituted by OH, OR 1 or NR 2 R 3, and which may be interrupted by one or more -O-, -S- or -NR 4 -; - when R is divalent, R is substituted or unsubstituted, linear or branched C 3 -C 18 alkylene, C 5 -C 7 cycloalkylene which can be substituted by OH, OR 1 or NR 2 R 3, and which may be interrupted by one or more -O-, -S- or - NR 4 -; - when R is trivalent, R is selected from N(CH 2 -CH 2 -*) 3, - R' is linear or branched C 1 -C 5 alkyl substituted by NHR 2, - B is a nitrogen containing organic base, - L is a C 1 -C 8 substituted or unsubstituted alkylene chain, or a substituted or unsubstituted C 6 cycloalkylene chain, or substituted or unsubstituted C 9 -C 11 polycyclic chain, or a substituted or unsubstituted double bond, which can be interrupted by O, S, NR 4, or a substituted or unsubstituted C 3 to C 4 alkenylene chain, or a substituted or unsubstituted benzene ring, or substituted or unsubstituted naphthalene ring, - R 1 is H, C 1 -C 18 alkyl, - R 2 and R 3, independently are hydrogen, unsubstituted or substituted C 1 -C 18 alkyl or phenyl; or R 2 and R 3 together with the nitrogen atom to which they are bonded form a 5 or 6 membered-ring which may contain a further heteroatom, - R 4 is H, unsubstituted or substituted C 1 -C 18 alkyl, - the ratio between A a and B a or between A b and B b ranges from 0.5: 1 to 10: 1. - n is 0 or 1 when X is a carbon atom, n is 2 when X is a sulfur atom, - n' is 0 or 1, - m is an integer from 1 to 6, - R is selected from monovalent, or divalent, or trivalent or polyvalent residues, wherein - when R is monovalent, R is H, OH, substituted or unsubstituted, linear or branched C 1 -C 18 alkyl, C 5 -C 7 cycloalkyl which can be substituted by OH, OR 1 or NR 2 R 3, and which may be interrupted by one or more -O-, -S- or -NR 4 -; - when R is divalent, R is substituted or unsubstituted, linear or branched C 3 -C 18 alkylene, C 5 -C 7 cycloalkylene which can be substituted by OH, OR 1 or NR 2 R 3, and which may be interrupted by one or more -O-, -S- or - NR 4 -; - when R is trivalent, R is selected from N(CH 2 -CH 2 -*) 3, - when R is monovalent, R is H, OH, substituted or unsubstituted, linear or branched C 1 -C 18 alkyl, C 5 -C 7 cycloalkyl which can be substituted by OH, OR 1 or NR 2 R 3, and which may be interrupted by one or more -O-, -S- or -NR 4 -; - when R is divalent, R is substituted or unsubstituted, linear or branched C 3 -C 18 alkylene, C 5 -C 7 cycloalkylene which can be substituted by OH, OR 1 or NR 2 R 3, and which may be interrupted by one or more -O-, -S- or - NR 4 -; - when R is trivalent, R is selected from N(CH 2 -CH 2 -*) 3, - R' is linear or branched C 1 -C 5 alkyl substituted by NHR 2, - B is a nitrogen containing organic base, - L is a C 1 -C 8 substituted or unsubstituted alkylene chain, or a substituted or unsubstituted C 6 cycloalkylene chain, or substituted or unsubstituted C 9 -C 11 polycyclic chain, or a substituted or unsubstituted double bond, which can be interrupted by O, S, NR 4, or a substituted or unsubstituted C 3 to C 4 alkenylene chain, or a substituted or unsubstituted benzene ring, or substituted or unsubstituted naphthalene ring, - R 1 is H, C 1 -C 18 alkyl, - R 2 and R 3, independently are hydrogen, unsubstituted or substituted C 1 -C 18 alkyl or phenyl; or R 2 and R 3 together with the nitrogen atom to which they are bonded form a 5 or 6 membered-ring which may contain a further heteroatom, - R 4 is H, unsubstituted or substituted C 1 -C 18 alkyl, - the ratio between A a and B a or between A b and B b ranges from 0.5: 1 to 10: 1.