The thermal behavior of alkylamine-zinc bromide adducts,(RNH(2))(2) ZnBr2, has been studied by differential scanning calorimetry, powder X-ray diffractometry, and broad-line H-1 NMR spectrometry. An enantiotropic mesophase is observed for the adducts of heptylamine and higher homologous members. Enthalpy change at the transition into an isotropic liquid located between 136 to 156 degrees C is as small as 2 to 3 kJ mol(-1). The X-ray diffraction pattern of the mesophase is of the unstructured smectic type. The layer spacing (d/nm) measured just above the melting point is approximated by d = 0.186 n + 0.597, where n is the number of carbon atoms in the alkyl group R, suggesting a bilayer structure. The second moment of the resonance spectrum in the mesophase is 0.1 to 0.3 G(2), indicating almost liquid-like characteristics. Equilibrium relations were determined for a number of binary systems composed of the adducts and aromatic compounds. Phenanthrene and acenaphthene dissolve into the mesophase of the octadecylamine adducts as much as 50 mol% but substantially less when the alkyl group is shorter.