Mazut is a heavy, low quality fuel oil, used in generating plants and similar applications. In the United States and Western Europe, Mazut is blended or broken down, with the end product being diesel.

Mazut may be used for heating houses in the former USSR and in countries of the Far East that do not have the facilities to blend or break it down into more conventional petro-chemicals. In the West, furnaces that burn Mazut are commonly called “waste oil” heaters or “waste oil” furnaces.