Soy Garlic Chicken

¼ C. soy sauce
¼ C. vegetable oil
2 T. Ketchup
1 T. vinegar
4 garlic cloves, minced
¼ tsp. pepper
4 boneless, skinless chicken breast halves, pounded out with a meat mallet

Combine first six ingredients in a shallow dish or zip-top plastic bag, add chicken.
Cover or seal and chill four hours, turning occasionally. Remove chicken from marinade, discard marinade.
 Grill over medium heat 3-6 minutes a side. Do not overcook.