Preheat oven to 300 degrees F. Place popcorn, pretzels, pecans, peanuts and cereal in a very large bowl; set aside.
Heat butter, brown sugar and corn syrup in a medium saucepan. Stir mixture over medium heat until it begins to boil. Boil 3 minutes without stirring. Remove from heat and stir in baking soda (mixture will foam).
Pour syrup over popcorn mixture in bowl and stir until evenly coated. Pour mixture onto a large, rimmed baking sheet or roasting pan. Bake 30 minutes, stirring twice during baking time. Stir mixture a few times as it cools on baking sheet. Store in an airtight container.