The ingredients (as written in bold) are listed on Australian cans of Coke Zero. Water anyone?
Carbonated water
Colour (150d) - aka "Sulphite Ammonia Caramel"
Food acid 338 - aka "Phosphoric Acid" (Also used for rust removal!)
Food acid 331 - aka "Sodium Citrates" (Also used in photography and as an anticoagulant of blood stored for transfusion!)
Flavour
Sweetener 951 - aka "Asapartame" (According to the US FDA, "the [National Cancer Institute] currently is studying aspartame and other dietary factors as part of a larger study of adult brain cancer." You might want to wait for the results before you drink it. Also, avoid this one if you're moody!)
Sweetener 950 - aka "Acesulfame Potassium" (Looking for reasons to avoid this sweetener? Look no further...)
Preservative 211 - aka "Sodium Benzoate" (Undiluted this stuff isn't too bad... Just don't let it touch your skin!)
Caffeine
http://www.thezeromovement.org/coke_zero_ingredients.html