Rod-like and spheric mercury sulfide (HgS) microcrystallites with hexagonal and cubic structure have been synthesized at room temperature by treating HgO with sulfur in ethylenediamine (en). Real time monitoring of both the UV-vis absorption spectrum and Raman spectrum during the dissolution process of sulfur in en solvent have been exploited for the first time to explore the underlying reaction mechanism. The characteristic peaks in absorption and Raman lines originated from the polysulfide ions S-6(2-) and S-3(-) are observed in the sulfur-en solution, which are found to decrease in concentration and lifetime with dissolution time, and become undetectable finally. The structure and morphology of the synthesized HgS microcrystallites change with the lifetime of the S-6(-) and S-3(-) ions in the solution. It is further observed that S-6(2-) and S-3(-) ions are responsible for the formation of hexagonal structure and spheric-shape morphology, while the en solvent is beneficial to the formation of rod-like single-crystalline HgS. (C) 2004 Elsevier Ltd. All fights reserved.