The Ba/Yb/Mg/Si intermetallic system is extremely complex, with four competing structurally related compounds forming from reactions of barium, ytterbium, and silicon in magnesium-rich Mg/Al flux. In addition to the previously reported Ba2Yb0.9Mg11.1Si7, Ba5Yb2Mg17Si12, and Ba20Yb5Mg61Si43, a new compound has been found. Ba6Yb1.84Mg18.16Si13 cstallizes in the (P6) over bar space group, with the Zr6Ni20P13 structure type. Quenching experiments and in situ neutron powder diffraction studies were carried out to determine the reaction parameters that favor particular products. Under slow-cooling conditions, Ba5Yb2Mg17Si12 precipitates from the flux at 800 degrees C. A faster cooling rate of an identical reaction results in the formation of single crystals of Ba20Yb5Mg61Si43 in the flux at 640 degrees C. This indicates that the crystallization of products in this metal flux reaction does not involve precipitation and interconversion of different phases but instead depends on the rate of cooling across the supersaturated metastable zone in this system.