In a Dutch oven, combine sugar and water; bring to a boil. Add citrus slices; reduce heat to medium. Cook until slices are translucent, about 20 minutes, turning occasionally. Reduce heat; simmer until slices are tender but still intact, about 10 minutes, turning occasionally.
Using a slotted spoon or tongs, remove slices to a wire rack or a parchment-lined baking pan. Let stand at room temperature overnight to dry. (Save syrup for another use.) If desired, cut slices in half; use to decorate a cake, pie or other dessert, or to garnish drinks.