Iron Sulfide (FeS) is a black, water-insoluble solid with the chemical formula FeS. It is one of a group of compounds and minerals containing iron in +2 (ferric) and sulfur in -2 (sulfide) valence states. It is the primary component of the mineral pyrrhotite and also occurs in magnetkies, troillite, and ferrochrome. It is an extremely toxic compound and may cause irritation to the skin, eyes, lungs, and gastrointestinal tract upon contact. It is also a very volatile substance that will spontaneously ignite in air, producing a poisonous vapor and irritating the respiratory system.

The solubility of iron sulfide is affected by the pH of the solution, the concentration of sulfur dioxide, and the presence of other reducing agents. Iron sulfide is soluble in acidic solutions and moderately soluble in alkaline solutions. It is insoluble in nitric acid. It is moderately soluble in petroleum hydrocarbons, but it reacts rapidly with sulfates to form metallic iron sulfides. It is also slightly soluble in sodium hydroxide solution.

Pyrophoric iron sulfide scale, powder, and sludge can build up on equipment used to handle corrosive sulfur compounds such as sour crude oil, high-sulfur fuel oils, and aromatic tars. When exposed to oxygen, it forms an exothermic reaction that can ignite the hydrocarbons and other materials in the tank, vessel, or pipeline. This can result in a fire, explosion, or other damage to the equipment. This hazard can be mitigated by using a high-pressure water stream to dissolve the material.