Here are the ingredients in Diet Coke, per McDonalds’ website:
Carbonated water, caramel color, phosphoric acid, sodium saccharin, potassium benzoate (to protect taste), natural flavors (vegetable source), citric acid, caffeine, potassium citrate, aspartame, dimethylpolysiloxane.
Scary stuff. Definitely not what I would call natural. I have heard about the danger of sodium benzoate, so I thought I’d check out the similar Diet Coke ingredient, potassium benzoate. Here’s what Wikipedia says about it:
In combination with ascorbic acid (vitamin C), sodium and potassium benzoate may form benzene, a known carcinogen. Heat, light and shelf life can affect the rate at which benzene is formed. The Food and Drug Administration (FDA) is currently (as of March 2006) performing tests, but the Environmental Working Group is calling for the FDA to publicly release all tests and use their authority to force companies to reformulate to avoid the potential benzene forming combination.
Potassium Benzoate (or E212) was recently described by the Food Commission, who campaign for ’safer, healthier food in the UK’, as “mildly irritant to the skin, eyes and mucous membranes”. 
Wow. Does that mean adding a lemon slice to Diet Coke makes it even worse? So I have officially not had a sip of the evil juice for a whole 8 days now. I will soon post a ticker to track my success with staying off it.