During coal gasification, the resulting slag varies in the mineral compositions and amount due to the ambient gas in gasifier. In this study, the microstructure and leaching behavior of typical slag were investigated under simulated mildly/strongly reducing gasification conditions with different H2S concentration. XRF, XRD and SEM-EDX were used to study the properties of slag. And ICP-AES was used to determine the metallic ions in acid-digested slag samples. The results indicate that iron phase was separated from molten slag phase in melting process under a strongly reducing condition. In the strongly reducing condition with H2S, FeSx(x < 1) was formed below about 1200 degrees C and it decomposed to iron liquid when the temperature is above about 1200 degrees C while it takes a long time to reach equilibrium. In the strongly reducing condition, heavy metals, such as Cr, Cu, Ni, Co easily form an interstitial compound in iron phase. Adding of H2S causes heavy metals enrichment in the iron phase, where it has a significant impact on Ni and Cu. On the other hand, addition of H2S has negligible effect on slag properties under mildly reducing condition. (C) 2013 Elsevier Ltd. All rights reserved.