The add in ingredients to the mix are 1/3 cup water, 1/3 cup vegetable oil and 1 egg. You can easily substitute out 1/4 cup pureed soft silken tofu or 1 egg equivalent Ener-G egg replacer for the egg in the mix. Both ways come out great. You will need to increase the cooking time by about 2-3 minutes (you can tell it's done when a toothpick passes through clean).
Aside from the phenomenal taste, texture and ease of veganizing this brownie mix has the best directions of any mix I've seen. It not only has directions for metal pan baking but glass pan baking as well. I only have glass so for me this is a huge help as I'm always left with the task of trial and error trying to translate baking times to fit the pan I'm using.
There's also a great recipe for Seven Layer Brownie Bars on the back of the box, which can also easily be veganized by trading out Earth Balance margarine for the butter and oat milk for the sweetened condensed milk to make yummy Seven Layer Brownie Bars.