Ultrathin two-dimensional bismuth chalcogenide materials have received substantial research attention due to their potential applications in electronics and optoelectronics. While solvothermal synthesis is considered to be one of the most promising methods for large-scale production of such materials, the mechanisms that govern the crystallization during solvothermal treatment are still poorly understood. In this work, the solvothermal syntheses of Bi2SexTe3−x (x = 0−3) hexagonal nanoplatelets were monitored by synchrotron-based in situ powder X-ray diffraction, which enabled investigation of crystallization curves, lattice parameters, and crystal size evolution under a variety of synthesis conditions. On the basis of the crystallization curves and crystal size evolution, a general 3-step crystallization process has been deduced: (1) An induction period for the dissolution of the precursor and nucleation of Bi2SexTe3−x, followed by (2) rapid growth of planar crystals through the oriented attachment, and finally (3) a diffusion-controlled slow growth step consuming the remaining precursor from the solution. Oriented attachment is very effective for the growth of binary composites, resulting in a high yield of large planar crystals; however, it is much less effective for the growth of ternary composites due to lattice mismatch of the nuclei formed at the induction period, producing much smaller crystals accompanied by a limited yield of large planar crystals. Additionally, three intermediate phases (Bi2TeO5, Bi2 SeO5, and Na2SeO3) were observed that played an important role in controlling the phase separation as well as the composition of the final ternary compounds.