Abstract— We present experimental evidence that demonstrates directly how the subcellular localization and redistribution of two nile blue derivatives, 5-ethylamino-9-diethyl-ami-nobenzo[α]phenothiazinium chloride (EtNBS) and 5-ethylamino-9-diethyl-aminobenzo[α]phenoselenazinium chloride (EtNBSe), affect oxygen consumption during irradiation of sensitized multicell EMT6 spheroids. Specifically, two well-defined phases of oxygen consumption are observed during treatment, with the onset of the second phase being a fluence-dependent event. Fluorescence microscopy during irradiation of EtNBS-sensitized EMT6 monolayer cultures indicates that sensitizer redistribution from intracellular organelles, presumably lysosomes, to the cytosol can explain the onset of the second oxygen consumption phase. This event requires eight times fewer photons for EtNBSe than for EtNBS, consistent with the higher singlet oxygen yield of the former dye. The existence of a second oxygen consumption phase suggests that the aggregated form of the dye is a less efficient photodynamic agent. Moreover, we present evidence suggesting that damage to the primary sites of localization might be less significant than damage incurred by the sites to which the sensitizer redistributes during irradiation.