Stir the broth, ketchup, oil, thyme, garlic powder and black pepper in a shallow, nonmetallic dish or gallon size resealable plastic bag. Add the beef, mushrooms and green pepper and toss to coat. Cover the dish or seal the bag and refrigerate for 30 minutes.
Remove the beef and vegetables from the marinade. Thread the beef, mushrooms and green pepper alternately on 4 skewers.
Lightly oil the grill rack and heat the grill to medium. Grill the kabobs for 20 minutes or until the beef is cooked through and the vegetables are tender, turning and brushing often with the marinade. Discard any remaining marinade.