The present invention is directed to novel salts of thieno[2,3-d]pyrimidine derivatives, including 4-(2-fluorophenyl)-6-methyl-2-(piperazin-1-yl)thieno[2,3-d]pyrimidine salts. The present invention is also directed to compositions including such polymorphs and methods for using such salts, e.g., in the treatment of gastrointestinal and/or genitourinary disorders.