Singlet Oxygen Sensor Green is a detection reagent that is highly selective for singlet oxygen. Unlike other available fluorescent and chemiluminescent singlet oxygen detection reagents, Singlet Oxygen Sensor Green does not show any appreciable response to hydroxyl radical or superoxide. This indicator initially exhibits weak blue fluorescence, but in the presence of singlet oxygen, it emits a green fluorescence (excitation/emission maxima ∼504/525 nm) similar to that of fluorescein. Singlet Oxygen Sensor Green is cell-impermeant.