Line a 13x9" baking pan with aluminum foil, spray with cooking spray; set aside. In a large mixing bowl, measure 10 cups cereal; set aside.
In a heavy-bottomed large pot, melt margarine over medium-low heat, stirring continuously. As soon as the margarine begins to turn brown, remove pan from heat and quickly stir in marshmallows. Combine marshmallows and margarine, then return pan to stove for about 2 minutes, or just until melted and almost smooth.
Stir cereal into melted marshmallow mixture and quickly toss to coat. Spread mixture into prepared pan and use a greased spatula to press firmly and evenly in pan. Allow Treats to set at room temperature for two hours before slicing and serving. Store Treats in an airtight container at room temperature for up to 1 week, or in the freezer for up to 3 months.