Delavirdine mesylate is a non-nucleoside inhibitor of the reverse transcriptase enzyme found in human immunodeficiency virus type 1 and is indicated for treatment of this infection. Several solid-state forms of delavirdine mesylate have been identified and characterized. Crystallization of delavirdine mesylate from refluxing methanol solutions using acetone as a co-solvent produced either form VIII (U-90152S) or XI (U-90152T) depending on the amount of co-solvent added. Both were anhydrous, non-hygroscopic polymorphs useful in solid formulations. Cooling supersaturated drug solutions in methanol below room temperature before adding acetone produced form XII, a solvate. Factor analysis of infrared spectra for delavirdine mesylate was used to classify research and production samples by solid-state form and quantitate polymorphic mixtures. Spectra from prepared mixtures of forms VIII or XII in form XI were quantitated by principal component regression analysis with standard errors for prediction of 2% and detection limits at 3–5% for both forms.