Most heroin base samples from actual cases have carbonyl absorbance maxima at 1761.3 and 1741.1 cm with additional differences in the spectra at 280 to 3200 cm and in the fingerprint region. These differences could not be accounted for as a result of impurities, decomposition products, additives, or diluents. At least two polymorphic forms of heroin base exist with various crystal habits. The main parameters determining which polymorph and crystal habit are obtained are the temperature conditions and solvent used for crystallization. (Author abstract)