The work described in this thesis is concerned with the study of extending known crystal engineering principles to the influence of anions on crystal packing and reactivity. Some features arising from this study include: single-crystal to single-crystal reactivity; an important role of water in modifying the course of the reaction; the movement of water associated with the reacting pair of acridizinium cations during the photodimerization process; possible martensitic phase transition from the acridizinium bromide monohydrate P2<SUB>1</SUB>/a isomer to P1 isomer; phase transformations accompanying dimerization; the generation of crystalline solutions incorporating two or more anions and the modification of structure as a function of anion with consequential differences in reactivity. Results from both theoretical approach and experimental studies on acridizinium and 9-methylacridizinium salts suggest that ionic molecules pack preferrentially in a centrosymmetric manner favoured by both the geometrical close-packing model and Coulombic energy. Crystal morphologies are modified by varying anions. The morphological changes can be correlated consistently with the size and shape of the anion, and the electronic property of the anion also contributes to this change (e.g. the formation of hydrogen bond). Molecular packing arrangements (structural relationship) of these compounds can also be correlated consistently with the size, shape and the electronic properties of the anions. There is a good correlation between the photoreactivity and the crystal structure. The relative conversion rate, assuming that the excimer formation is the rate determining step, can be predicted reasonably well from the ground state structure of the monomer pair. A possible mechanism for the photodimerization of acridizinium and 9-methylacridizinium salts was proposed based on the results from the electrochemical, spectroscopic and electron resonance studies.