To reduce refractory damage of a basic oxygen furnace, precise control of slag composition during blowing is important. In many cases, an interfacial layer is formed and the formation and decomposition rates of this layer are considered to be rate-controlling steps. In this research, the influence of the interfacial layer on the dissolution rate is investigated by performing fundamental experiments. The reagents were mixed in various proportions and sintered to make an oxide mixture that simulated the interfacial layer. The sintered sample was added to a molten slag stirred by Ar gas, and the dissolution rate was determined from the change in slag composition. The results revealed the importance of the effects of the density and composition of the interfacial layer on the dissolution rate.