The search for materials capable of storing small molecular species is experiencing a shift from solids with permanent porosity towards organic materials capable of the uptake and release of low-molecular-weight guests. We demonstrate that a solid mixture of the pharmaceutical compound lamotrigine with a range of saturated and unsaturated 1,4-butanedicarboxylic acids, when in combination with a third molecule, can result in the formation of a family of isostructural materials involving a structurally persistent binary-host framework based on a hydrogen-bonded molecular salt of lamotrigine and the acid. A systematic study, based on mechanochemical screening, has revealed a remarkable robustness to subtle changes in the chemical functionality of the acid in that at least 12 different acids can be used in combination with lamotrigine to generate isostructural binary-host frameworks. Such robust isostructurality results in the important attribute that the shape, size and surface chemistry of the inclusion cavities can be fine-tuned by systematic variation of the substituents on the dicarboxylic acid.