(S)-(-)-2,6-Diamino-4,5,6,7-tetrahydrobenzothiazole was alkylated with 1-propyl p-toluenesulfonate in dimethylformamide as solvent for 96 hrs. at room temp. The heterogeneous reaction system did not dissolve (only the nature of the ppt changed) and ensured min. polyalkylates. The resulting pramipexol tosylate was collected by filtration, dried in vacuo at 35 degrees C. and converted into the monohydrous or an-hyd. dihydrochloride. The method is cheap.