Lay the chicken fillets on a chopping board and slice each in half horizontally. Place in a large, shallow ovenproof dish, in which the fillets fit snugly.
Mix together the soy sauce, sugar, garlic, vinegar, orange juice and pepper and pour the mixture over the chicken. Cover and chill the dish until you are ready to cook it.
Uncover the dish and bake the chicken in a preheated oven, 180°C (350°F), Gas Mark 4, for 30 minutes, until it is cooked through. Transfer to serving plates and spoon the cooking juices over the meat.
Serve with steamed vegetables and rice or noodles.