Process for preparing polyols from a composition A comprising one or more compounds of formula wherein R 1, R 2, R 3, R 4, R 5, R 6 and R 7 are, independently of one another, and for R 3 and R 7 independently for each of the n units, chosen from the group consisting of H, -OR 15, C 1 -C 10 alkyl which is unsubstituted or substituted with one or more -OR 15 groups, C 6 -C 12 aryl which is unsubstituted or substituted with one or more -OR 15 groups, or C 3 -C 10 cycloalkyl which is unsubstituted or substituted with one or more -OR 15 groups, or a group of formula (Ia) R 15 represents H or C 1 -C 10 alkyl which is unsubstituted or substituted with one or more -OH groups; a, b, x and y are independently of one another, independently for each group of formula (Ia), independently for each [(CH 2) a -C=C-(CH 2) b] r unit and independently for each [(CH 2) x -C=C-(CH 2) y] p unit, an integer between 0 and 20, advantageously between 0 and 15, preferably between 0 and 12; r is an integer between 1 and 10, advantageously between 1 and 5; p is an integer between 1 and 10, advantageously between 1 and 5; n is an integer between 1 and 7; said process comprising a step a) of placing together, with stirring and under an atmosphere of hydrogen and of carbon monoxide: • at least one precatalyst which is a complex comprising a transition metal chosen from column 9 of the periodic table, • a tertiary amine, or a non-quaternary ammonium salt thereof, of formula NR 8 R 9 R 10 wherein R 8, R 9 and R 10 represent, independently of one another, a C 1 -C 10 alkyl, a C 6 -C 12 aryl or a C 3 -C 10 cycloalkyl, or R 8 and R 9 form, together with the nitrogen atom to which they are attached, a heterocycle comprising four, five or six ring members, • said composition A comprising said compound of formula. R 1, R 2, R 3, R 4, R 5, R 6 and R 7 are, independently of one another, and for R 3 and R 7 independently for each of the n units, chosen from the group consisting of H, -OR 15, C 1 -C 10 alkyl which is unsubstituted or substituted with one or more -OR 15 groups, C 6 -C 12 aryl which is unsubstituted or substituted with one or more -OR 15 groups, or C 3 -C 10 cycloalkyl which is unsubstituted or substituted with one or more -OR 15 groups, or a group of formula (Ia) R 15 represents H or C 1 -C 10 alkyl which is unsubstituted or substituted with one or more -OH groups; a, b, x and y are independently of one another, independently for each group of formula (Ia), independently for each [(CH 2) a -C=C-(CH 2) b] r unit and independently for each [(CH 2) x -C=C-(CH 2) y] p unit, an integer between 0 and 20, advantageously between 0 and 15, preferably between 0 and 12; r is an integer between 1 and 10, advantageously between 1 and 5; p is an integer between 1 and 10, advantageously between 1 and 5; n is an integer between 1 and 7; said process comprising a step a) of placing together, with stirring and under an atmosphere of hydrogen and of carbon monoxide: • at least one precatalyst which is a complex comprising a transition metal chosen from column 9 of the periodic table, • a tertiary amine, or a non-quaternary ammonium salt thereof, of formula NR 8 R 9 R 10 wherein R 8, R 9 and R 10 represent, independently of one another, a C 1 -C 10 alkyl, a C 6 -C 12 aryl or a C 3 -C 10 cycloalkyl, or R 8 and R 9 form, together with the nitrogen atom to which they are attached, a heterocycle comprising four, five or six ring members, • said composition A comprising said compound of formula. • at least one precatalyst which is a complex comprising a transition metal chosen from column 9 of the periodic table, • a tertiary amine, or a non-quaternary ammonium salt thereof, of formula NR 8 R 9 R 10 wherein R 8, R 9 and R 10 represent, independently of one another, a C 1 -C 10 alkyl, a C 6 -C 12 aryl or a C 3 -C 10 cycloalkyl, or R 8 and R 9 form, together with the nitrogen atom to which they are attached, a heterocycle comprising four, five or six ring members, • said composition A comprising said compound of formula.