Pour the margarita mix into a large resealable plastic bag. Add the chicken breasts, and press out as much of the air as possible before sealing. Marinate 4 to 8 hours in the refrigerator.
Preheat an outdoor grill for medium-high heat.
Lightly oil the grill grate. Remove chicken from marinade, and discard the marinade. Grill chicken 8 minutes per side, or until juices run clear.
