In May 1979, Diamond Shamrock announced that it would be moving its corporate headquarters from the crime-stricken Cleveland, Ohio to Dallas, Texas in an effort to improve the company’s image and reputation.

[6] Looking to expand its presence, the USD$160 million acquisition of the Sigmor Corporation in July 1982 added 600 more stores to Diamond Shamrock’s portfolio of locations and also as a dividend, it received ownership of the Three Rivers oil refinery located between San Antonio and Corpus Christi, Texas.

[7] February 1987 saw the detachment of Diamond Shamrock’s production and exploration branch and would rename itself to the Maxus Energy Corporation once the split from its refining and marketing division took effect.

[9] In September 1996, it was announced that Diamond Shamrock would be acquired by Montreal-based Ultramar, a Canadian oil company for USD$1.96 billion.

The primary reason for the merger was originally to attain USD$75 million in annual savings, according to Roger Hemminghaus, the chairman of the newly combined companies Ultramar Diamond Shamrock.

Prior to this, the conversion of Diamond Shamrock stores had already started with the east and west coasts of the United States.

[15] The Diamond Shamrock brand is currently being used in Colorado, California, Georgia, New Mexico, Oklahoma, Texas, amongst other states.

After World War II Ray Evans, TR's son, led the company to decentralize its operations.

During the 1950s a third plant was constructed in Muscle Shoals, Alabama, helping the company continue to enlarge its range of products, expanding to produce plastics and chemicals for agriculture.

James Harold Dunn joined the company in 1938 as a vice president and general manager, having previously been an engineer at the Lone Star Gas Corporation.

During 1939-1940 Lone Star and Shamrock cooperated on the construction of a plant at Murchison in Henderson County whose aim was to recycle natural gas.

In 1955, Claybourne Allison "Buzz" Cash succeeded him, and in 1959 Shamrock opened its first catalytic cracking unit in Sunray.

In 1978, Sigmor purchased its stations back from Diamond Shamrock, continuing to market DS products.

gave Diamond Shamrock 600 retail outlets, plus the Three Rivers oil refinery which was built by Sigmor during the 1970s.

Kit Freiden of the Associated Press stated "some analysts predicted the giant energy company would reject the offer.

That year, Diamond Shamrock bought the National Convenience Stores Stop N Go chain for $260 million.

[19] In 1996, Canadian company Ultramar bought Diamond Shamrock for $1.96 billion in stock and assumed debt.