Left: the ratio of (S+ + S++)/(O+ + O++) as a function of the ionization fraction of oxygen, O+/O for H II regions in NGC 5457. We also include the ionization corrections fractions derived by Thuan et al. (1995) and Kennicutt et al. (2003). Right: the ratio of (S+ + S++)/(O+) as a function of the ionization fraction of oxygen.