As part of McDonald's Canada's ongoing transparency campaign, the company's executive chef Dan Coudreaut responds to a customer's question about the Big Mac's "special sauce" by demonstrating how to make your own using commonly found ingredients.
According to Coudreaut, an analogous sauce can be easily prepared using mayonnaise, sweet pickle relish, yellow mustard, white wine vinegar, garlic powder, onion powder, and paprika.
Serious Eats dug up a list of Big Mac sauce ingredients found on the McDonald's Canada website, and they are effectively identical to those used in Coudreaut video, with four exceptions: xanthan gum (a food thickening agent), potassium sorbate (a food preservative), and calcium disodium EDTA (another preservative).
The moral of this story? Make your Big Macs at home.