A comprehensive study of crystal morphology and imperfections in the isomorphic series potassium acid phthalate-rubidium acid phthalate (KAP-RbAP) in aqueous solutions was conducted using a general model of the processes of mixed crystal formation in solutions discussed earlier by the authors on experimental and theoretical basis. The system was chosen as a model to interpret basic processes. The utilization of both substances as well as their solid solutions was taken into account. Interaction between RbAP or KAP crystals and aqueous solutions saturated with either KAP or RbAP or both, in ratios varying from 10 to 90 wt% (the step was about 10 wt%), has been studied. All the processes were observed in situ and ex situ by optical microscopy and X-ray topography (Lang method). Solubilities of KAP and RbAP mixtures in water were determined and compositions of corresponding solid solutions were estimated for the isotherms 29degreesC and 39degreesC. These indicated a considerable convex curvature in concentration Schreinemakers diagrams. The main peculiarity of crystals is their having heteroepitaxial porous or uniform textures, which occur due to the secondary volume deficit or excess at the exchange reaction between a crystal and solution. In turn, the primary/secondary volume ratio depends on the solubilities of the components in the system (unconnected with the molar volumes of the substances) determined directly from Schreinemakers diagrams. The convexity of the KA-P-RbAP solubility isotherms displays specific crystal formation in this system compared with features of Co-Ni Tutton salts whose isotherms have small curvatures. Firstly, this appeared as a shift from volume-deficit replacement toward volume-excess replacement. This resulted in non-uniform heteroepitaxial textures and sharp slowing down of the process at an intermediate stage. Phenomena occurring in such peculiar systems and interactions between the crystals and solutions of intermediate compositions have not been investigated before. The results obtained became a basis for developing a theory of mixed crystal formation in solutions. Optically transparent mixed monocrystals containing about 3 and 5-10 wt% RbAP were isolated from solutions having the KAP/RbAP ratios of components 90/10 and 80/20 wt%. (C) 2003 Elsevier Science B.V. All rights reserved.