Claims

1. A process in which iodixanol is prepared in a series of successive processes by dimerisation of 5-acetamido-N,N'-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide ("Compound A") in which, after the dimerisation step, unreacted Compound A isprecipitated from the reaction mixture and recovered, optionally crystallised, and then re-used in a subsequent process in the series.

2. A process according to claim 1 in which the dimerisation step is carried out using epichlorohydrin, 1,3-dichloro-2-hydroxypropane or 1,3-dibromo-2-hydroxypropane as the dimerisation agent in a non-aqueous solvent or in water or a mixture of water and one or more alcohols.

3. A process according to claim 2 in which the dimerisation agent is epichlorohydrin and the solvent is 2-methoxyethanol or methanol.

4. A process according to claim 1 in which precipitation of Compound A is effected with water, optionally together with an alcoholic co-solvent.

5. A process according to claim 4 in which the pH of the mixture is adjusted to 10-11 with acid to provoke precipitation, the temperature adjusted if necessary to 15-40 C and the solution optionally seeded with crystals of Compound A.

6. A process according to claim 5 in which further acid is added to a pH of 2-5.

7. A process according to claim 1 in which the Compound A recovered is recrystallised.

8. A process according to claim 1 in which, after separation of Compound A, the iodixanol-containing mixture is purified without the use of chromatographic methods.

9. A process according to claim 1 in which the recovered Compound A is re-used in a subsequent process for the preparation of iodixanol.