Cut chicken breasts into thin strips.

In a medium bowl, combine sugar and cornstarch. Gradually stir in vinegar and soy sauce. Stir in pineapple, green pepper and onion, set aside.

Heat oil in wok. Add chicken and stir fry, just until tender, and chicken has turned white. Add pineapple and vegetable mixture; stir until well mixed. Cover and simmer over low heat 15 minutes.