Chop chicken into bite sized pieces.
Heat oil in skillet and cook chicken over medium heat until nearly done.
Add onion, green pepper, red pepper and garlic.
While chicken cooks, stir together reserved pineapple juice, vinegar, brown sugar, soy sauce and cornstarch.
Finish cooking until chicken is cooked through and onion is soft.
Add pineapple to the skillet.
Pour in sauce, cooking until bubbly and thick.