The invention provides a crystalline sulfate salt of 3-endo-(8-{2-[cyclohexylmethyl-((S)-2,3-dihydroxy-propionyl)amino]ethyl}-8-aza-bicyclo[3.2.1]oct-3-yl)benzamide or a solvate thereof. The invention also provides pharmaceutical compositions comprising such crystalline salt forms, methods of using such crystalline salt forms to treat diseases associated with mu opioid receptor activity, and processes useful for preparing such crystalline salt forms.本發明提供3-內-(8-{2-[環己基甲基-((S)-2,3-二羥基-丙醯基)胺基]乙基}-8-氮雜-雙環[3.2.1]辛-3-基)苄醯胺或其溶劑合物之結晶硫酸鹽。本發明亦提供包含此等結晶鹽形式之醫藥組合物、使用此等結晶鹽形式來治療與μ類鴉片受體活性相關之疾病的方法,及適用於製備此等結晶鹽形式之方法。