BODIPY™ 581/591 undecanoic acid can be used to detect reactive oxygen species (ROS) in cells and membranes. Oxidation of the polyunsaturated butadienyl portion of the dye results in a shift of the fluorescence emission peak from ∼590 nm to ∼510 nm. We recommend dissolving in high-quality anhydrous DMSO to attain a stock concentration of 1 to 10 mM. In reduced state, the excitation and emission maxima of BODIPY™ 581/591 undecanoic acid is 581/591 nm; after oxidation, the probe shifts the excitation and emission to 488/510 nm.