The invention relates to crystalline forms of 4-((R)-2-{[6-((S)-3-methoxy-pyrrolidin-1-yl)-2-phenyl-pyrimidine-4 carbonyl]-amino}-3-phosphono-propionyl)-piperazine-1-carboxylic acid butyl ester hydrochloride, processes for the preparation thereof, pharmaceutical compositions comprising said crystalline forms, pharmaceutical compositions prepared from such crystalline forms and their use as a medicament, especially as a P2Y12 receptor antagonist.