Novel solvates and crystal polymorphs of Ritonavir are disclosed, as well as methods of making them. Specific solvates of the compound include a formamide solvate and a partially desolvated solvate. Also disclosed are methods of making previously known forms of Ritonavir. Methods of using the novel forms of Ritonavir for the treatment of diseases, such as HIV-infection, are disclosed, as are pharmaceutical compositions and unit dosage forms comprising the novel forms of Ritonavir.