The key to understanding the synthesis and thermal stability of the kesterite lies with the governing chemical equilibria, the steady‐state conditions of processing, and the mass‐transfer mechanisms. This chapter discusses the identification of secondary phases in terms of common laboratory equipment as well as synchrotron‐based methods. It also talks about the kesterite sulfide system in particular, but exactly the same considerations apply for the selenide and mixed chalcogen systems. Importantly, the findings shown are not limited to electrodeposition and annealing (EDA) but are material properties independent of the fabrication route, and therefore relevant to all research in the field of kesterite synthesis. The chapter examines the chemistry of the kesterite formation and decomposition reactions using the theory of chemical equilibria. It presents a brief overview of solid‐gas equilibrium reactions. Finally, the consequences of the kesterite equilibrium are discussed in terms of synthetic strategies to maximize the kesterite phase.