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Spatial and temporal small-angle X-ray scattering (SAXS) scans show that 8.6 and 11.8 nm iron oxide nanoparticles (NPs) in heptane drop-cast on top of a heptane layer atop a diethylene glycol (DEG) layer are trapped at the DEG/heptane interface to generally form a single ordered, hexagonal-close packed monolayer (ML), and this occurs long before the heptane evaporates. Many NPs remain dispersed in the heptane after this NP assembly. Assembly occurs faster than expected from considering only the diffusion of NPs from the drop-cast site to this liquid/liquid interface. The formation of the ordered NP ML occurs within 100 s of drop-casting, as followed by using the (10) ordered NP SAXS peak, and on the same time scale there is a concomitant decrease in the SAXS form factor from disordered NPs that is apparently from disordered NPs at the meniscus. Usually, most of the ordered NPs are close packed, but there is evidence that some are ordered although not close packed. After the heptane evaporates, a close-packed ordered NP ML remains at the DEG/vapor interface, though with smaller NP–NP separation, as expected due to less van der Waals shielding caused by the upper medium in the interface. X-ray beam transmission at different vertical heights characterizes the heptane and DEG bulk and interfacial regions, while monitoring the time dependence of SAXS at and near the DEG/heptane interface gives a clear picture of the evolution of NP assembly at this liquid/liquid interface. These SAXS observations of self-limited NP ML formation at the DEG/heptane interface are consistent with those using the less direct method of real-time optical reflection monitoring of that interface.