The continuous isomorphic exchange between different parts of a crystal surface, which involves alternating spots of simultaneous dissolution and growth, is mediated by a solution and determines formation of such crystals, being a cause of unordinary phenomena. Modified concentration diagrams allow determination of the pathways and volume effects of exchange reactions as well as interpretation of growth kinetics and morphology of crystals. Data obtained by in situ observing and measuring crystallization effects of soluble salts comprising a wide spectrum of isomorphic compounds provide the basis for the development of an experimental-theoretical methodology. Characteristic features of phase equilibria and mixed crystal formation in ternary solutions induced by a continuous isomorphic exchange between the phases are presented. Growth and exchange phenomena are analyzed by phase diagrams. The proposed concept constitutes an important part of fundamental methodology of crystal growth and synthesis of mixed crystals.