Layered alkali-containing 3d transition-metal oxides are of the utmost importance in the use of electrode materials for advanced energy storage applications such as Li-, Na-, or K-ion batteries. A significant challenge in the field of materials chemistry is understanding the dynamics of the chemical reactions between alkali-free precursors and alkali species during the synthesis of these compounds. In this study, in situ high-resolution synchrotron-based X-ray diffraction was applied to reveal the Li/Na/K-ion insertion-induced structural transformation mechanism during high-temperature solid-state reaction. The in situ diffraction results demonstrate that the chemical reaction pathway strongly depends on the alkali-free precursor type, which is a structural matrix enabling phase transitions. Quantitative phase analysis identifies for the first time the decomposition of lithium sources as the most critical factor for the formation of metastable intermediates or impurities during the entire process of Li-rich layered LiLi0.2Ni0.2Mn0.6O2 formation. Since the alkali ions have different ionic radii, Na/K ions tend to be located on prismatic sites in the defective layered structure (Na2/3-xNi0.25Mn0.75O2 or K2/3-xNi0.25Mn0.75O2) during calcination, whereas the Li ions prefer to be localized on the tetrahedral and/or octahedral sites, forming O-type structures. Structure changes of a mixture of alkali-free precursor and alkali species during the synthesis of layered Li-, Na-, or K-containing 3d transition-metal oxides (ATMOs) were monitored by in situ high-resolution synchrotron-based X-ray diffraction. The intermediate phases, contributing to the ATMO formation pathway, were directly observed, which provide valuable information for the rational design and synthesis of advanced layered oxides with desirable structural and chemical properties. Display omitted •In situ high-resolution HT-sXRD techniques was used to unveil the Li/Na/K-ion insertion induced structural evolution during heating.•The dynamics of chemical reaction between alkali-free precursor and alkali species upon calcination were systematically investigated.•High-temperature lithiation reaction pathway strongly depends on the alkali-free precursor type.•Site preferences of Li/Na/K-ion leads to the formation of various types of layered structures.