Creosote is a highly flammable, oily substance produced by tars during the burning of wood. When burning wood, especially wet or overly dry wood, some of the unburnt wood particles will go up the chimney and accumulate on the sides of the chimney along with the carbon contained in the smoke. Water vapor is a by-product of fire and the combination of carbon, unburnt wood particles, and water vapor, create creosote. Creosote is highly flammable and that is why it is important to have your chimney cleaned regularly.