Nevirapine (C 15 H 14 N 4 O) is a lipophilic drug of low aqueous solubility used in AIDS treatment. Three different crystal forms of this non-nucleoside reverse transcriptase inhibitor were obtained after recrystallization procedures. Physical characterization was evaluated with DSC, TG, IR spectroscopy, and X-ray diffraction methods. Two new pseudopolymorphs have been characterized: nevirapine hemihydrate and nevirapine hemiethyl acetate. The pseudopolymorph identified as the hemiethyl acetate is disordered. It is converted to the hemihydrate form at room temperature. The crystal structures of the hemihydrate and hemiethyl acetate forms have been determined. The hemihydrate crystallizes in space group P2 1 /n, while the ethyl acetate form is in space group P1 h. Polymorphs and pseudopolymorphs of a drug may exhibit different chemical and physical properties, which can affect dissolution, besides manufacturing, stability, and bioavailability. For this reason, an investigation on the behavior of the two nevirapine pseudopolymorphs through the dissolution test has been described. The disk intrinsic dissolution rate of the solvent-free form has been found to be 1.5-fold greater than the hemihydrate form in 0.01 mol L -1 HCl and in 0.1 mol L -1 HCl, which can be explained by solubility results.