The present invention relates to an improved method for the synthesis of the (S)- or (R)-enantiomer of omeprazole, characterized in that 2-[[(4-X-3,5-dimethylpyridin-2-yl)methyl]thio]-5-methoxy-1H-benzimidazole or 2-[[(4-X-3,5-dimethyl-1-oxidopyridin-2-yl)methyl]thio]-5-methoxy-1H-benzimidazole, wherein X is a leaving group, is oxidized into the corresponding sulphoxide which is obtained as a crystalline compound. Recrystallization of the thus obtained sulphoxide results in a compound of enhanced chemical and optical purity, which is subsequently transformed into the (S)- or (R)-enantiomer of omeprazole.