An investigation of the mullite decomposition mechanism and kinetics in the temperature range 1900-2000 degrees C was conducted. SEM-EDS and XRD analysis showed that the decomposition of mullite in this temperature range was primarily caused by the presence of carbon from furnace components, A reaction mechanism, in which gas-phase SiO2 reacts with carbon to form SiO and CO, is proposed; both IR analysis and kinetic studies support the proposed mechanism. The decomposition was completely suppressed by the introduction of a modified crucible assembly and the addition of excess SiO, to the reaction system. A mullite melt of stable composition was thus obtained, setting the groundwork for inviscid melt spinning mullite fiber making process development.