Marine sediment can function both as a source and as a sink of marine chemical contaminants. The toxicity of contaminated marine sediment can be assessed by toxic evaluation of its pore water, the inter-particle water of sediment, because toxicants in the pore water may be bioavailable to marine organisms. In this study, the toxicity identification evaluation (TIE) was performed to identify the major toxicants in the pore water of marine sediment collected in Hong Kong waters. In Phase 1 TIE, the suspected toxicants were characterized as anions or organic compounds that are either oxidizable or filterable in alkaline medium. In Phase 2 TIE, the suspected toxicants were identified as sulfide (S(2-)) based on the reduction of toxicity due to lowering of sulfide concentrations by experimental manipulations. The mass balance and spiking analyses in Phase 3 confirmed that S(2-) was one of the major toxicants and that some non-toxic unknown compounds measured by LC-MS, which was removed by C18 solid phase extraction, enhanced the toxicity of S(2-) in the pore water samples.