Line your counter with sheets of wax paper and lightly mist with non-stick cooking spray.

In large bowl, mix popcorn, pretzels and Chex. Set aside.

In medium to large saucepan, bring butter, brown sugar and corn syrup to a bubble over medium high heat. Keep sugar mixture bubbling {not violently, but not weak...some where in the middle} for 5 minutes, stirring occasionally. Remove from heat and stir in baking soda. Immediately pour over popcorn mixture and stir to coat evenly. Pour out onto greased wax paper and spread out. Sprinkle M&M's onto warm caramel munch mix so they stick. Let cool completely {10-15 minutes should do it}. Break into smaller pieces and store in air tight containers.