Okay, I found it. I also found this:
Pesach products are generally made with white distilled vinegar which is specially-made without any chametz or kitnios concerns. However, some companies choose to substitute glacial acetic acid for the vinegar in their Pesach products. Glacial acetic acid is chemically identical to vinegar, but is made from petroleum feedstock as opposed to being fermented, and poses little Pesach concern.
What is petroleum feedstock? Sounds gross.