In a medium bowl, mix soy sauce, vegetable oil, sherry, brown sugar, and garlic. Pour into a large resealable bag. Place chicken in the bag, and shake to coat. Marinate in the refrigerator at least 4 hours.
Preheat an outdoor grill for high heat, and lightly oil grate.
Place chicken on the prepared grill. Cook 6 to 8 minutes on each side, or until no longer pink and juices run clear. Discard remaining marinade.
