Abstract: A fire suppression system for use with ducts carrying corrosive gases has a multiplicity of sprinkler assemblies, each having an easily replaceable adapter with a sprinkler head having a fusible alloy fill. The head is screwed into one end of the adapter which has an integral flange at its opposite end. The adapter is inserted in a flanged nozzle which is part of the duct. The adapter flange is bolted to the nozzle flange which has a central opening adapted for connection to a source of extinguishing fluid. A continuous passage for fluid is provided through the adapter and the flanges together with a passage through the sprinkler head, the latter being opened when the system is activated. Only the face of the adapter and a small protruding portion of the sprinkler head are exposed to the corrosive gases. The sprinkler head has a corrosion-resistant body and has a central passage containing a seal plug and a fusible fill at opposite ends and a piston between the fill and the plug.