Recent breakthroughs in Ge1−xSnx and SiyGe1−x−ySnx materials and devices have created much excitement within the photonics research community. Group IV semiconductors with true direct bandgaps have now been realized, and they hold much promise for advancing integrated photonics technologies. Epitaxial growth is the primary enabling factor in this research field; however, an understanding of the fundamental surface and interfacial phenomena associated with this is still lacking. Due to the metastable nature of these alloys, low growth temperatures must be employed and it is still not understood how the chemical precursors achieve sufficient reactivity at such low temperatures. In this work, the authors discuss the chemical mechanisms responsible for the epitaxial growth of Ge1−xSnx alloys during chemical vapor deposition with GeH4 and SnCl4. They first describe how the growth rate and layer composition varies with precursor flow rates and temperature and then discuss the Arrhenius behavior and incorporation efficiency of each respective precursor. Finally, they connect these observed trends to a thermochemical analysis of likely reaction pathways. The authors find that exothermic gas-phase reactions likely produce reactive intermediates which allow the reaction to proceed at unexpectedly low temperatures.