The invention is concerned with (S)-1-{[2-(5-Methyl-2-phenyl-oxazol-4-yl)-ethylamino]-acetyl}-pyrrolidine- -2-carbonitrile mesylate and crystalline polymorphs of this compound. This compound and its polymorphic forms exhibits superior properties compared to the previously known compounds and can be used as medicament for the treatment of disorders which are associated with DPP-IV.