PDMPO, a lysosome pH indicator, is an excellent fluorescent acidotropic reagent for fluorescence imaging. PDMPO is a potent tool with which to study acidic organelles of live cells. PDMPO exhibits pH-dependent dual-excitation and dual-emission spectral peaks. PDMPO produces a blue fluorescence in weakly acidic organelles and shifts to yellow in more acidic lysosomes. (Abs=329 nm; Em=440/540 nm) [1].