SynopsisAlkali-metal oxides and fluorides were substitutionally added into a commercial mold flux currently used in China Steel Corp. to examine their effects on the flux viscosity. The solution mechanism of alkali-metal oxides and that of fluorides into the molten flux were discussed. The correlations between polymerization coefficient (K) and the break points, viscosities and activation energies were presented. It was found that the effect of all the alkali-metal oxide and fluoride additions on the flux viscosity continually became less pronounced with increasing temperatures. And based on the result of this study, the additions of Li20 and Na2SiFs to the mold flux could be the best choices, provided a low-viscosity and low-break point flux was desired.