X-Ray crystal structure of the hydrochloride salt of an antipsychotic drug blonanserin (BLN) and characterization of a high temperature polymorph are reported. The crystal structure of BLN HCl monohydrate was determined. All new solid-state forms are characterized by X-ray diffraction, thermal microscopy, ss-NMR, and FT-IR spectroscopy. BLN HCl transformed to its monohydrate form during solubility measurement. The solubility of BLN and BLN HCl monohydrate in 60% EtOH–water is 1.6 g L⁻¹ and 119.2 g L⁻¹ respectively.