To control the content of free CaO in the molten slag produced by municipal solid waste melting, experimental studies were conducted to quantify the dissolution of CaO particles into molten slag. The present work focused on the effect of Fe and Mg contents on the dissolution rate of CaO particles into simulated molten slags of CaO-SiO2-Al2O3 containing various amounts of FeO and MgO in the temperature range of 1673-1723 K. Results showed that, in comparison with FeO-free molten slag, the dissolution rate of CaO into molten slags containing FeO was significantly enhanced and was proportional to the FeO content. It was also shown that the presence of MgO in most cases enhanced the dissolution rate of CaO, but no appreciable relationship was observed between the dissolution rate and the MgO content. The apparent rate constant of dissolution of CaO particles into the molten slag was determined from the experimental data with the assumption that the mass transfer of CaO through molten slag film was the rate-determining step. It was found that the apparent rate constant of dissolution of CaO increased linearly with increasing FeO content. This apparent rate constant could be used to predict the time required for complete dissolution of CaO into actual molten slags.