Real-time reassembly of an ordered nanoparticle monolayer clue to UV-photolysis of the surfactant shell of nanoparticles was observed. The technique of grazing-incidence small-angle X-ray scattering provided the possibility to track in situ the nanoparticle pair correlation function of die sample processed in a UV-ozone reactor. The analysis revealed a total shift of similar to 1 nm of the nanoparticle nearest-neighbor distance. The temporal evolution of the interparticle distance proved to he the first-order process governed by the UV-photolysis and described by a single-exponential decay Function. The nanoparticles tend to agglomerate into a labyrinth-like structure with a typical length scale of some 30 nm.