A method for producing all-E astaxanthin dimethyldisuccinate of the formula I which comprises the process steps: a) reacting a C 10 dialdehyde of the formula II in a double Wittig reaction with a C 15 phosphonium salt of the formula III where X - is the anion of an organic or inorganic acid HX, in a solvent comprising dichloromethane by adding a solution of an alkali metal alkoxide or alkaline earth metal alkoxide M 1 OR 1 or M 2 (OR 1) 2 in an alcohol R 1 OH as base, where M 1 is Li, Na, K or Rb, M 2 is Mg, Ca, Sr or Ba and R 1 is methyl, ethyl, n-propyl or isopropyl, to give an astaxanthin of the formula IV wherein the two new double bonds formed are not only present in the Z configuration but also in the E configuration, optionally followed by an aqueous workup of the reaction mixture for removing the salts M 1 X or M 2 X 2; b) removing the alcohol R 1 OH from the reaction mixture of process step a) by repeated washing of the reaction mixture with water or by distilling off a solvent mixture comprising dichloromethane and the alcohol R 1 OH, optionally followed by drying the reaction mixture by removing water by means of azeotropic distillation of dichloromethane and water; c) reacting the astaxanthin of the formula IV which is present in the reaction mixture treated in process step b) in dichloromethane by adding methyl succinoyl chloride of the formula V and an organic nitrogenous base to form an astaxanthin dimethyldisuccinate of the formula Ia; d) an aqueous workup of the reaction mixture of process step c) for removing water-soluble salts; e) solvent exchange of dichloromethane by an alcohol R 2 OH by removing the dichloromethane from the reaction mixture of process step d) by distillation and adding the alcohol R 2 OH to the reaction mixture, wherein R 2 is methyl, ethyl, n-propyl or isopropyl; f) thermal isomerization of the astaxanthin dimethyldisuccinate of the formula Ia produced in process step c) to give the all-E astaxanthin dimethyldisuccinate of the formula I by heating the R 2 OH-comprising reaction mixture of process step e) to at least 50°C, wherein the reaction mixture has a water content of 10 to 90% by volume of water based on the total volume of the reaction mixture, and wherein the all-E astaxanthin dimethyldisuccinate of the formula I occurs in crystalline form and is subsequently optionally isolated. a) reacting a C 10 dialdehyde of the formula II in a double Wittig reaction with a C 15 phosphonium salt of the formula III where X - is the anion of an organic or inorganic acid HX, in a solvent comprising dichloromethane by adding a solution of an alkali metal alkoxide or alkaline earth metal alkoxide M 1 OR 1 or M 2 (OR 1) 2 in an alcohol R 1 OH as base, where M 1 is Li, Na, K or Rb, M 2 is Mg, Ca, Sr or Ba and R 1 is methyl, ethyl, n-propyl or isopropyl, to give an astaxanthin of the formula IV wherein the two new double bonds formed are not only present in the Z configuration but also in the E configuration, optionally followed by an aqueous workup of the reaction mixture for removing the salts M 1 X or M 2 X 2; M 1 is Li, Na, K or Rb, M 2 is Mg, Ca, Sr or Ba and R 1 is methyl, ethyl, n-propyl or isopropyl, b) removing the alcohol R 1 OH from the reaction mixture of process step a) by repeated washing of the reaction mixture with water or by distilling off a solvent mixture comprising dichloromethane and the alcohol R 1 OH, optionally followed by drying the reaction mixture by removing water by means of azeotropic distillation of dichloromethane and water; c) reacting the astaxanthin of the formula IV which is present in the reaction mixture treated in process step b) in dichloromethane by adding methyl succinoyl chloride of the formula V and an organic nitrogenous base to form an astaxanthin dimethyldisuccinate of the formula Ia; d) an aqueous workup of the reaction mixture of process step c) for removing water-soluble salts; e) solvent exchange of dichloromethane by an alcohol R 2 OH by removing the dichloromethane from the reaction mixture of process step d) by distillation and adding the alcohol R 2 OH to the reaction mixture, wherein R 2 is methyl, ethyl, n-propyl or isopropyl; f) thermal isomerization of the astaxanthin dimethyldisuccinate of the formula Ia produced in process step c) to give the all-E astaxanthin dimethyldisuccinate of the formula I by heating the R 2 OH-comprising reaction mixture of process step e) to at least 50°C, wherein the reaction mixture has a water content of 10 to 90% by volume of water based on the total volume of the reaction mixture, and wherein the all-E astaxanthin dimethyldisuccinate of the formula I occurs in crystalline form and is subsequently optionally isolated.