Two polymorphs (Forms I and II) of the antitubercular drug isoxyl (N,N′-[4-(3-methylbutoxy)phenyl]thiourea) have been identified and characterized by powder and single crystal X-ray diffraction, thermal analysis (hot stage microscopy and differential scanning calorimetry), Fourier transform infrared spectroscopy and 13C solid-state nuclear magnetic resonance spectroscopy. Single crystal X-ray diffraction revealed that the two polymorphs contain different conformers of isoxyl whose hydrogen bonding schemes and packing arrangements in their respective crystals are different. Thermal analysis led to the conclusion that the two polymorphs are enantiotropically related, with a transition temperature of 91.7 °C. A schematic energy-temperature diagram was subsequently constructed. In addition, aqueous solubility data for the two polymorphs were measured, enabling a comprehensive characterization of this dimorphic system.