The invention relates to salt forms of (3R,4S)-1-((4-amino-5H-pyrrolo[3,2-d]pyrimidin-7-yl)methyl)-4-(methyl-thiomethyl)pyrrolidin-3-ol, as well as polymorphic forms of the salts. The invention further relates to processes for preparing the salt forms and to the use of the salt forms in the treatment of diseases and disorders where it is desirable to inhibit 5&prime-methylthioadenosine phosphorylase (MTAP).