The dissolution rate of dense magnesia specimen in calcium aluminate based melts was measured in air over the temperature range of 1450–1600°C, using a rotating disk/cylinder technique. The measured dissolution rates were strongly dependent on the rotation speed with the results indicating mass transfer in the slag phase to be the rate-limiting step. At a given rotation speed, the dissolution rate was strongly dependent on the slag chemistry and temperature. The diffusivity of MgO in the slag was calculated from the dissolution rate and solubility data, using known mass transfer correlations. Addition of FeO_x and CaF₂ and increasing the temperature, resulted in substantial increase in the dissolution rate and deduced diffusivity of MgO in the slag.