Preheat oven to 350 degrees. Prep baking dish with nonstick cooking spray.
Mix ketchup, brown sugar, vinegar, Worcestershire sauce, and mustard together in a small bowl.
Put the chicken breasts in the baking dish, Stir the other ingredients. Cover the chicken breasts with the sauce.
Bake at 350 degrees for 40 minutes. (Leave the baking dish uncovered) Allow to cool slightly before serving. Serve.