[5] Tesoro was an independent refiner and marketer of petroleum products, operating ten refineries in the Western United States with a combined rated crude oil capacity of approximately 1,200,000 barrels (190,000 m3) per day.

The first segment involved the operation of six petroleum refineries situated in California, Alaska, Washington, Hawaii, North Dakota, and Utah.

[22] Major pollutants emitted annually by the corporation were estimated to include more than 400,000 lb (180 t) of sulfuric acid,[23] following which the Environmental Protection Agency named Tesoro a potentially responsible party for at least four superfund toxic waste sites.

Tesoro gave over $1 million in support of California Proposition 23, which aimed to suspend the Global Warming Solutions Act of 2006.

[27] Tesoro bought Libyan crude oil, after the overthrow of Colonel Gaddafi, in early April 2011 from the Vitol Group to supply its then-Hawaiian refinery.