The present invention relates to a process for stereoselective synthesis of substituted sulfoxides either as a single enantiomer or in an enantiomerically enriched form. Thus, 5-methoxy-2-[[(4-methoxy-3,5-dimethyl-2-pyridinyl)methyl]thio]-1H-benzimidazole is reacted with (R)-camphorsulfonyl chloride to form a mixture of 1-(R)-camphorsulfonyl-5- (and 6-)methoxy-2-[(3,5-dimethyl-4-methoxy-2-pyridyl)methylthio]-1H-benzimidazole, oxidized to obtain a diastereomeric excess of 1-(R)-camphorsulfonyl-(5- and 6-)-methoxy-2-[(3,5-dimethyl-4-methoxy-2-pyridyl)methyl-(S)-sulfinyl]-1H-benzimidazole over 1-(R)-camphorsulfonyl-(5- and 6-)-methoxy-2-[(3,5-dimethyl-4-methoxy-2-pyridyl)methyl-(R)-sulfinyl]-1H-benzimidazole, the diastereomers are separated by fractional crystallization and the separated 1-(R)-camphorsulfonyl-(5- and 6-)-methoxy-2-[(3,5-dimethyl-4-methoxy-2-pyridyl)methyl-(S)-sulfinyl]-1H-benzimidazole is deprotected to give esomeprazole.