1. Preheat a grill for medium-high heat. Lay out eight squares of heavy-aluminum foil. Place one piece of chicken in the center of each square.
2. Pour the teriyaki sauce over the chicken, turning to coat. Distribute equal amounts of the peppers, onion, mushrooms and pineapple on top of each chicken breast. Fold the foil and seal tightly into packets.
3. Place the packets on grill, and cook for about 20 minutes, or until chicken is no longer pink and juices run clear. Check chicken by taking one packet off the grill and check before removing them all.