To elucidate the mechanism of photosensitization with pulsed light excitation, we previously introduced fluorescence-oxygen diagram that shows the correlation between photochemical oxygen consumption and photobleaching during a treatment (Kawauchi et al., Photochem. Photobiol., 80, 216-223, 2004). In pulsed photodynamic treatment of A549 cells with ATX-S10•Na(II), the diagrams for treatments at relatively high repetition rates of 10 and 30 Hz showed the complex behaviors of photochemical reaction; photobleaching initially occurred with oxygen consumption but it was switched to oxygen-independent photobleaching, which was followed by a secondary oxygen-consuming regime. In this study, fluorescence microscopy revealed that for treatments at 10 and 30 Hz, subcellular fluorescence distribution of ATX-S10•Na(II) changed drastically from the high-intensity spotty patterns showing lysosomal accumulation to the diffusive patterns within the cytosol during certain ranges of total light dose. These ranges were found to coincide with those in which oxygen-independent reaction appeared. These findings suggest that the sensitizer started to be redistributed from lysosomes to the cytosol during the oxygen-independent reaction regime. On the other hand, at 5 Hz, such reaction switching was not clearly seen during whole irradiation period in the diagram; this was consistent with the observation that sensitizer redistribution efficiently occurred even in the early phase of irradiation. The appearance of oxygen-independent reaction at the higher repetition rates may be caused by high local concentration of the sensitizer and the resultant low concentration of oxygen in the reaction sites due to the shorter pulse-to-pulse time intervals. In pulsed photodynamic treatment, pulse frequency is an important parameter that affects the intracellular kinetics of the sensitizer and hence the photochemical reaction dynamics.