Heat oven to 300 degrees. In large bowl, combine popcorn, pretzels, pecans, peanuts, and cereal squares; set aside.
In medium saucepan over medium heat, heat butter, brown sugar, and corn syrup. Stir mixture until it begins to boil. Boil 3 minutes without stirring. Remove from heat and stir in baking soda. (Note: Mixture will foam.)
Pour syrup over popcorn mixture and stir until evenly coated. Pour mixture onto large rimmed baking sheet or roasting pan. Bake 30 minutes, stirring twice during baking; remove from oven.
Stir mixture several times as it cools. Store in airtight container.