The high temperature viscosity of the TiO₂-MnO-Al₂O₃-8.64ZrO₂-2.77Na₂O welding flux system was measured by the rotating spindle method to identify the relationship between the viscosity and melt structure at various compositions of TiO₂, MnO, and Al₂O₃ contents. At temperatures of 1773 K to 1748 K and fixed TiO₂/MnO ratio, the effect of Al₂O₃ on the viscosity was not significant, but slightly increased with higher Al₂O₃. At temperatures below 1748 K, the effect of Al₂O₃ was more pronounced with increments of Al₂O₃ significantly increasing the viscosity of the molten flux. Increased extended basicity ((TiO₂/+1.13MnO)/SiO₂) depolymerized the network structure, where TiO₂ and MnO works to depolymerize the present melt. Raman analysis of as-quenched oxide melts from 1773 K showed the symmetric [AlO₄]-tetrahedral stretching vibrations to increase and the asymmetric [AlO₆]-octahedral stretching vibrations to decrease with higher concentration of Al₂O₃ at various fixed TiO₂/MnO ratios suggesting polymerization of the structure with Al₂O₃ additions. The opposite trend could be observed with increasing extended basicity. XPS (x-ray photoelectron spectroscopy) results showed the bridged oxygen (O^o) to increase and the non-bridged oxygen (O^–) to decrease with Al₂O₃ additions and lower extended basicity also suggesting polymerization of the network structure in the present melt system.