Fluxes are added to steelmaking process to form basic slag and limit refractory lining consumption.Early formation of CaO- and MgO-saturated slag is critical for process productivity. To achieve this,flux must be dissolved in the melt in a limited process time, and the properties of a slag additiveinfluence thefluxing efficiency. The dissolution behavior of samples with different qualities (i.e.raw dolomite, limestone, soft-burnt and hard-burnt quicklime and dolime) was investigated in thepre-melted model BOF and EAF slags at 1673 K and 1723 K. The disc-shaped sample wasimmersed into the melt and left there until slag solidification was reached. After a holding timebetween 5 and 20 minutes, the crucible with the sample were quenched with liquid nitrogen. Thefused samples were analyzed by SEM method. Based on comparison of slag morphology andconcentration changes in the slag, the dissolution behavior of the samples was examined.