The features of hydrothermal crystallization of reaction mixtures RM-I and RM-II that are similar in chemical composition and differ in the order of mixing of the reactants, have been studied in detail. It has been shown that changing the sequence of mixing the reactants during the preparation of the reaction mixture (RM) leads to the formation of aluminosilicate solid gels of different compositions and determines the formation mechanism of the BEA zeolite crystalline structure. It has been established that the addition of a source of silicon at the initial step of mixing the reactants (RM-I) leads to the formation of an aluminum-rich aluminosilicate gel, the charge of which is compensated by alkali metal cations, with the TEA(+) cations occurring in solution. Adding a source of aluminum at the initial steps of mixing the reactants (RM-II) leads to the formation of amorphous aluminosilicate hydrogel with occluded TEA(+) cations, the chemical composition of which is close to that of the final zeolite. It has been shown that during crystallization of RM-I the formation of nuclei apparently occurs in solution. According to the infrared spectroscopy data, during crystallization of RM-II the formation of secondary structural fragments of BEA zeolite occurs in the bulk of the solid phase. The products of separated hydrothermal transformation of solid and liquid phases isolated by centrifugation from RM-I and RM-II at the initial steps of the synthesis have been studied. It has been demonstrated that in order to obtain zeolite Beta crystals without admixtures of other phases it is necessary to have not only a high concentration of TEA(+) cations, but also a high concentration of aluminum in the reaction mixture.