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-100 is a fuel oil that is manufactured to GOST specifications, for example GOST 10585-99. Mazut is almost exclusively manufactured in the Russian Federation, Kazakhstan, Azerbaijan, and. The most important thing when grading this fuel is the sulphur content.

Mazut 100-75 VLS and Mazut 100-99 Grade I are actually the same thing. GOST merged the old classifications of 75 and 99 into a new seven grade classification, all under М100-99. For whatever reason, many people still use the old 75 classification; particularly the Chinese.

The grades are represented by these sulphuric levels:

• "Very Low Sulphur" is mazut with a sulphur content of 0.5%

• "Low Sulphur" is a mazut with a sulphur content of 0.5-1.0%

• "Normal Sulphur" is a mazut with a sulphur content of 1.0-2.0%

• "High Sulphur" is a mazut with a sulphur content of 2.0-3.5%.