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Determination of the iron oxide nanoparticles (IONPs) local temperature during laser heating is important in the aspect of laser phototherapy. We have carried out theoretical modeling of IONPs local electromagnetic (EM) field enhancement and heating under the laser action near individual IONPs and ensembles of IONPs with different sizes, shapes and chemical phases. For experimental determination of IONPs temperature, we used fluorescence thermometry with rhodamine B (RhB) based on its lifetime. Depending on the IONPs shape and their location in space, a significant change in the spatial distribution of the EM field near the IONPs surface is observed. The local heating of IONPs in an ensemble reaches sufficiently high values; the relative change is about 35 °C for Fe2O3 NPs. Nevertheless, all the studied IONPs water colloids showed heating by no more than 10 °C. The heating temperature of the ensemble depends on the thermal conductivity of the medium, on which the heat dissipation depends. During laser scanning of a cell culture incubated with different types of IONPs, the temperature increase, estimated from the shortening of the RhB fluorescence lifetime, reaches more than 100 °C. Such "hot spots" within lysosomes, where IONPs predominantly reside, lead to severe cellular stress and can be used for cell therapy. [ABSTRACT FROM AUTHOR]