Abstract: The present invention describes an improved process for the preparation, isolation, and purification of the anti-ulcer agent omeprazole whereby the sulfide precursor pyrmetazole is reacted subsurfacely with exactly one molar equivalent of meta-chloroperoxybenzoic acid in methylene chloride or toluene solution; residual organic solvent is removed from the aqueous layer by vacuum distillation; crude product is obtained by reactive crystallization with an alkyl formate and seeding; and pure product is isolated by recrystallization in methanol-water containing aqueous NaOH by subsurface addition of aqueous acetic acid to pH 9.0, seeding, filtration, washing, and drying. Compositions of omeprazole containing no chromatographically detectable levels of residual non-alcoholic organic reaction solvent are also described.