Chlorine is made primarily by the electrolysis of sodium chloride. Underground deposits of fairly pure sodium chloride are found in several areas, and these are the result of the evaporation of trapped seas from some 200 million years ago. In the UK the most accessible deposits are in Cheshire, Lancashire, Cleveland and Northern Ireland, though they extend under the North Sea as far as Poland.

Sodium chloride can be mined as rock salt (for example at Winsford, UK), but most is extracted by dissolving in water under pressure and pumping to the surface. Sodium carbonate is used to precipitate calcium and other metal ions, and other reagents are used to remove suspended particles. High-purity salt solution is required for most electrolytic cells.

Electrolysis requires electrical energy. A mercury cell typically operates at only 4.5v, but draws a current of 300kA. A single facility may consist of 100 such cells, drawing a total current of 300MA (million amps). Part of the environmental impact of chlorine production is related to the method used for electricity generation.