Refining is a complex operation that depends upon the human skills of operators, engineers and planners in combination with cutting edge technology to produce the products that meet the demands of an intensely competitive market.

Some of the greatest challenges for the refinery in recent years have been changes to the specification of transportation fuels. The switch from leaded to unleaded petrol and, more recently, European legislation reducing sulphur levels in diesel and petrol required major investment in new and existing refinery units.

At its simplest, oil refining is the distillation of crude oil into different fractions. But many further complex processes are necessary to produce a full range of products that includes: propane and butane (LPG), petrol, jet fuel, diesel, marine fuels, heating oil, lubricant basestocks, fuel oil and bitumen.

The refinery also supplies feedstock to the adjacent ExxonMobil Chemical plant.

The catalytic cracking unit, known as the cat cracker, is one of the most important plants on the refinery. The cat cracker takes the heavier, less valuable molecules from the distillation process and breaks them down into smaller ones. These smaller, more useful and therefore more valuable molecules are used for manufacturing petrol and provide feedstocks for the chemical plant.

In one of the most technically challenging projects undertaken at an ExxonMobil refinery anywhere in the world, the cat plant's 50 year old reactor was recently replaced at a cost of £70 million. This investment enables the refinery to operate more efficiently and will  help to sustain the long-term profitability of the whole Fawley site.

Refining processes use substantial amounts of energy and there has been considerable investment in recent years to make the refinery more energy efficient. A £60 million combined heat and power (CHP) generating plant was installed in 1999, potentially reducing energy costs by more than £2 million per year. Using less energy brings the additional environmental benefits of reducing emissions of gases such as sulphur dioxide, carbon dioxide and nitrogen oxides.