In solid-state chemistry, stable phases are often missed if their synthesis is impractical, such as when decomposition or a polymorphic transition occurs at relatively low temperature. In the preparation of complex oxides, reaction temperatures commonly exceed 1000 degrees C with little to no control of the reaction pathway. Thus, a prerequisite for exploring the synthesis of complex oxides is to identify reactions with intermediates that are kinetically competent at low temperatures, as provided by assisted metathesis reactions. Here, we study the assisted metathesis reaction Mn2O3 + 2.2YCl(3)center dot 6H(2)O + 3Li(2)CO(3) -> 2YMnO(3) + 5.8LiCl + 0.2LiYCl(4) + 3CO(2) using in situ synchrotron X-ray diffraction. By changing the atmosphere, oxygen vs inert gas, the reaction product changes from the overoxidized perovskite YMnO3+delta to the hexagonal YMnO3 polymorph at the reaction temperature of 850 degrees C, respectively. Analysis of the reaction pathways reveals two parallel reaction pathways in forming YMnO3 phases: (1) the slow reaction of metal oxides in a LiCl flux (Y2O3 + Mn2O3 -> (6LiCl) 2YMnO(3)) and (2) the fast reaction from ternary intermediates (YOCl + LiMnO2 -> LiCl + YMnO3). Control reactions reveal that both proposed pathways in isolation result in product formation, but the direct preparation of ternary intermediates (YOCI + LiMnO2 -> LiCl + YMnO3) occurs at lower temperatures (500 degrees C) and shorter times (<24 h) and forms nominally stoichiometric orthorhombic YMnO3. These ternary intermediates react at a faster rate than the slow stepwise oxygenation of yttrium chloride to Y2O3 (YCl3 -> YOCl -> Y-3 O4Cl -> Y2O3), which is relatively inert. These results support a kinetically controlled reaction pathway to form YMnO3 phases in assisted metathesis reactions with phase selectivity achievable through changes to reaction atmosphere.