diet ketchup

Here's an old diet ketchup recipe.
It was originally a Shasta Cola recipe. I personally use Diet Rite Cola. 

Diet Ketchup 
1 cup good-quality ketchup
1 cup diet cola (I suppose quality works; so does cheap) 

Put ketchup and cola in bowl. Pour cola slowly less fizz is better. 
Stir the two ingredients. Do not whip! If it foams up, the flavor lessens. 
Now you have a decision to make. You can use it like it is and brush
it on as you cook. 
Or, you can cook it down and make it thick and use it anyway you want. 
I have never doubled this, but I see no reason not to.

Source: rec.food.recipes