A method for manufacturing hydrogen sulphate (alpha S) of the alpha-(2-chlorophenyl)-6,7dihydrothieno[3,2-c]pyridine-5(4H)-acetic acid methyl ester (clopidogrel hydrogen sulphate) of formula I, in crystalline Form I, wherein the compound of formula is separated out of a solution of clopidogrel in the form of the free base or salt in a solvent selected from the series of primary, secondary or tertiary C1-C5 alcohols, their esters with C1-C4 carboxylic acids, or optionally of mixtures thereof.