This study concernes with the reaction mechanisms of zeolite A and Y crystallization. Currently, there are two different mechanisms proposed:the solution transport mechanism and the solid phase transfor-mation mechanism. In an attempt to clarify this matter of mutually conflicting theories, three different types of experiments were carried out as follows. 1) Circulating reactions, in which clear solution is separated from solid gel and continually circulated through solid gel bed. 2) Separate reactions, in which clear solution and solid gel phases are separated and kept at reaction temperature in separate flasks. 3) Mixed reactions, in which the gel solution mixture is dept at reaction temperature in the conventional way. Samples of clean solution as well as solid gel were analyzed by XRD and IR for zeolite formation, and concentration changes of these samples were analyzed by I.C.P.A. From these experiments, the following conclusions were obtained. In crystallization of zeolite A, the predominating mechanism is the solid phase transformation, although some ions are consummed from the solution phase on incorporation in the solid phase reaction. In crystallization of zeolite Y, the aluminate ions in solution phase are transported to the solid phase until the Si/Al ratio reaches about 2, then the solid phase is transformed to zeolite Y crystals.