Zeolites comprise a rich class of microporous crystalline aluminosilicates: typically crystallized in nearly pure form from alkaline aluminosilicate aqueous solutions in batch reactors. It is well known that zeolite synthesis outcomes are sensitively dependent on the total batch composition, temperature, time and other initial and boundary conditions of the reaction system. Effects of the total batch composition on the nature of zeolite products are usually presented in the form of crystallization diagrams, but previous modeling efforts have not been able to explain structure of such diagrams. We calculate theoretical crystallization diagrams for synthesis of zeolites A and X under the assumption of pseudoequilibrium between the: two zeolite phases and a homogeneous solution. This approach allows us to analyze solubility effects separately from others, such as nucleation and competitive kinetics of crystal growth. First, we identify a solubility product of zeolite X using a thermodynamic-solution model that accurately represents speciation in zeolite mother liquors. As the silicon-to-aluminum ratio of zeolite X approaches the limiting value of I its solubility product approaches that of zeolite A. Then we calculate a theoretical crystallization diagram for zeolites A and X in the Na2O-SiO2-Al2O2H3O-H2O system. The results are ina very good agreement with experimental observations. This suggests that selective nucleation is not necessary to produce a pure zeolite product and that solubility considerations alone can explain observed crystallization diagrams in this particular case. Solubility based calculations using the present solution model can be used for steady-state design of reactors for synthesis of zeolites A and X when nucleation of desired phases is assured. A though we cannot yet conclusively discriminate between selective nucleation, growth kinetics and solubility to explain the product selectivity in particular zeolite producing systems, we present a modeling framework capable of such discrimination.