The most effective treatment of toxic ashes and slags from waste incineration is the melting process. Organic substances are destroyed and heavy metals partially vaporized. The product is a glassy material which is mostly resistant to leaching. Activity and mass-transfer coefficients of minority components in synthetic slags, necessary to calculate the vaporization process, were determined using a top-blowing reactor. Different experiments were carried out with varying slag composition, oxygen partial pressure, and temperature. Additionally, the slag leaching resistance was tested. The aim of the work was the development of a basis for a melting process operating calculation.