Ph.D. / The specific topic of this investigation is the crystal chemistry and thermotropic polymorphism of the inorganic mineral acid salts of the n-alkyldiamines. This series of compounds contain organic cations with a linear alkane backbone that ranges from short to long chains. As a result their properties changes according to chain length. As they are salts they have the ability to conduct current resulting in wide industrial applications, biological as well as in their use as surfactants and ionic liquids at relatively low temperatures. They also have promising properties for use as propellants, explosives and other pyrotechnic compositions. Their electrical and electronic properties are under investigation. Their ability to exhibit polymorphism has not previously been established. Polymorphism is the ability of a material to exist in more than one crystal form. Since long-chained materials are known to have flexible hydrocarbon chains, by chemical intuition it is reasonable to assume that these materials may have more than one type of molecular arrangement. Various investigations of the conformational and thermotropic polymorphism of the salts of the related n-alkylamines have previously been published in the literature but very little information is available for the polymorphism that may or may not exist in the salts of the n-alkyldiamines. This study is limited to the short and medium chain length n-alkyldiammonium halide salts; nitrate salts; sulphate and perchlorate salts where the general chemical formula for the materials under investigation is CnH2nN+H3X where n = 2 to 12 (except 11) and X = Cl-, Br-, I-, NO3 -, SO4 2- and ClO4 -. It was anticipated that within this range of materials both the hydrogen bonding interactions and the Van der Waal’s forces (as combined packing forces) will play a part in controlling the molecular packing. A large number of the materials were synthesized and their structural information was analyzed by the complimentary techniques of X-ray diffraction and thermal analysis. X-ray diffraction was used in this investigation as it is the best technique to study the crystal chemistry and the polymorphic behaviour of these materials. The results obtained from this method of analysis, the positions of atoms in the crystals, allows for the analysis of the three-dimensional packing in the crystal structure as well as the identification of the hydrogen bonding interactions. The technique of single crystal X-ray diffraction allows the determination of the crystal structures of polymorphs at specific temperatures resulting in information on the effect of change in temperature on crystal packing. Since the compounds investigated in this study were relatively easy to crystallize it was possible to obtain the large single crystals required to obtain single-crystal X-ray diffraction data, and accurate crystal structures could be obtained by this method. Out of a total of thirty-eight crystal structures compared in this investigation, sixteen were novel crystal structures and fourteen were rederminations of previously published structures deemed to be of inferior quality and not suitable for the comparative study. The remaining eight data sets were used as published. Two thermal analytical techniques, Differential Scanning Calorimetry (DSC) and Hot-Stage Microscopy (HSM), were employed to establish if thermotropic phase changes were evident in these materials. Phase transition temperatures at above ambient temperatures were determined by DSC methods while morphological and textural changes in crystals of the compounds under investigation were monitored by HSM methods. In many of the materials analyzed, multiple above ambient phases were identified and only one structure showed a unique below ambient solid-state phase transition.