The invention provides a method for preparing glutathionylcobalamin (GluSCbl) which involves running the reaction in an aqueous solvent with a relatively small excess of glutathione, i.e. GluSH, specifically from one to less than four molar equivalents of GluSH. The formed GluSCbl is precipitated from the aqueous solvent, preferably by the addition of a precipitate inducing solvent. This provides GluSCbl in acceptable purity without the need for an additional chromatographic purification step. Additionally, it has been found that the reaction can be run to an acceptable level of purity by using very high concentrations of the reactants, i.e. around saturation concentrations for aquocobalamin. This has the advantage of eliminating the need for air-free conditions.