Mazut is a heavy, low-quality fuel oil, used in generating plants and similar applications. Mazut is blended or broken down, with the end product being diesel. Mazut may be used for heating houses in countries of the Far East that do not have the facilities to blend or break it down into more conventional petrochemicals.

Furnaces that burn mazut are commonly called "waste oil" heaters or "waste oil" furnaces. Mazut-100 is a fuel oil that is manufactured to GOST specifications, for example, GOST 10585-75/99.

GOST is the Russian system of standards. Mazut is almost exclusively manufactured in the Russian Federation; This product is typically used for larger boilers in producing steam since the BTU content is high.

The most important consideration when grading this fuel is the sulfur content, which can mostly be affected by the source feedstock. Mazut is much like Number 6 Oil and is part of the products left over after gasoline and lighter components are evaporated from the crude oil.