Preheat the oven to 350 degrees. Lightly grease a 9-by-13-inch baking pan and set aside. (You can use the butter wrapper or nonstick spray.) Or, for easier cleanup, line the pan with parchment.
Pour the cereal, pecans and pretzels into a 3-quart or larger bowl or cooking pot and toss to mix.
In a small saucepan, begin melting the butter over medium-high heat. Stir in the brown sugar, and bring the mixture to a boil. Cook at a moderate boil for 1 minute. Remove from the heat and pour at once over the cereal mix. Toss lightly to coat the mix as well as possible.
Pour the cereal mix into the baking pan and bake, uncovered, for 8 minutes. Remove from the oven, stir gently, and then continue to bake 8 minutes more. Remove from the oven and allow the mix to cool to room temperature (in the pan), about 45 minutes to 1 hour.
Keeps for up to 2 weeks in an airtight container or tin.