A series of arylformamides and arylthioamides has been synthesized and analyzed using nuclear magnetic resonance spectroscopy (NMR), differential scanning calorimetry (DSC), powder and single crystal X-ray diffraction. The work involved the study of hydrogen bonding, weak intermolecular interactions, phase changes and co-crystallization in aryl - formamides and -thioamides resulting in the structure determination of twenty four crystals. Three sets of isomorphic compounds were identified from the 24 solid state structures: set one; 2,6-difluorophenylformamide (1a), 2,6- dichlorophenylformamide (2a) and 2-chloro-6-methylphenylformamide (4a); set two; 2,6-dimethylphenylthioamide (17) and 2-chloro-6- methylphenylthioamide (18) and set three; 2,6-diisopropylphenylformamide (6) and 2,6-diisopropylphenylthioamide (20). In the first two sets, 1a, 2a and 4a, and 17 and 18, there are similar regions of halogen interactions and hydrocarbon interactions with disorder in the chloro-methyl substituents in structures 4a and 18. As for compounds 6 and 20, both the chemical and geometrical effects (size and volume of the isopropyl substituents) play a role in their isomorphism. A mixture of 2,6-dichlorophenylformamide (2a) and 2,6- dimethylphenylformamide (3) yielded a co-crystal 22 in which there was one molecule in the asymmetric unit, same as co-crystal 23 [derived from 2,6- dichlorophenylthioamide (17) and 2,6-dimethylphenylthioamide (18)]. The molecules of the two co-crystals displayed disorder in the substituents on the 2 and 6 positions of the aryl ring as a result of the occurrence of chlorine and methyl groups in the same crystallographic sites. Co-crystal 22 adopted the structure of 2,6-dichlorophenylformamide 2a. Co-crystal 23 also had a ii structure similar to that of 2a and co-crystal 22. Co-crystal 24 derived from a mixture of 2,6-diisopropylphenylformamide (6) and 2,6-diisopropylphenylthioamide (20), and also had one molecule in the asymmetric unit which showed disorder in the position occupied by oxygen and sulfur atoms. The 24 structures studied exhibited a variety of motifs formed from weak intermolecular interactions. Investigation of these weak intermolecular interactions revealed four different categories1 for the arylformamides and only one category for the arylthioamides. The categories were different in their formation of N-H…O/S hydrogen bonds (in which adjacent molecules are related by 21-screw axes, glide planes or by translation) forming chains (as in category 1, 2 and 5), sheets (as in Category 3) or dimers and tetramers (as in category 4). The chains in categories 1, 2 and 5 are in the for form of spirals (molecules along the chain are related by 21-screw axes or glide planes) or stacks (molecules along the chains are related by translation). Compounds from the different categories had certain interactions that contributed most to the stabilizations of their crystals. Apart from the N-H…O/ S hydrogen bonds, π…π, C-H…π, C-F…π, C-H…F, C-H…Cl, C-H…O, Cl…Cl, Br…Br, Cl…O and Br…O interactions also had a role to play in the stabilization of the different structures. Lattice energies and the energies relating to different molecular arrangements were calculated using Gavezzottis’ OPIX program suit. This showed that the N-H…O/S hydrogen bonds and π…π interactions were the most important interactions amongst the 24 structures discussed in this work. The crystal structures, thermal behaviour and phase transformations of all arylformamides and arylthioamides have shown that a phase transformation was only observed when a halogen atom was one of the substituents and only for some of the formamides. 2,6-dichlorophenylformamide 2a and 2-chloro-6-methylphenylformamide 4a transform to a hightemperature form at 155 and 106 °C, respectively. The high-temperature forms 2b and 4b (grown by sublimation) are both monoclinic but not isomorphous, with one short axis of about 4.3 A°, and consist of chains of N– H…O hydrogen-bonded molecules stacked along the short axis, related by translation. 1a and 1b are related to the above polymorphs in their formation of N-H…O hydrogen bonding patterns. Finally, this contribution has analyzed the role of weak interactions on the structural and thermal properties of the compounds studied. In addition, a mechanism for the phase change in 2,6-dichlorophenylformamide has been proposed and rationalized through the examination of the structures themselves together with lattice energy calculations. (1. Category = different types of hydrogen bonding patterns formed by disubstituted phenyl -formamides and -thioamides discussed in this thesis).