Emulsions stabilized by solid particles are known as Pickering emulsions.Although Pickering emulsions are encountered in various natural and industrialprocesses such as crude oil recovery, oil separation, cosmetic preparation, and wastewater treatment, the underlying phenomena are poorly understood. Recently, there isan increasing interest in Pickering emulsions because they open new avenues ofemulsion stabilization.Pickering emulsions, composed of droplets of one immiscible liquid in anotherliquid stabilized by solid particles, were discovered almost a century ago.1 In addition toincreasing possibilities for practical applications, Pickering emulsions also provide anew and convenient experimental model system for investigating solid particles atliquid-liquid interfaces.2,3 The advantages of using Pickering emulsions include, (i) aPickering emulsion generates a well-defined three-phase system that providesopportunities to evaluate the multiphase interactions of nanoparticles; (ii) a Pickeringemulsion is formed by self-assembly of solid particles at the emulsion interface; thus,the self-assembled structure directly reflects the balance between attractive andrepulsive forces between the particles; (iii) the confinement at the interface restricts the3D Brownian movement, and makes it more convenient for direct visualization ofparticle diffusion at the 2D; (iv) easily changeable interfacial properties such ascurvature, tension, and rheology.In our work, using confocal microscopy and environmental transmissionmicroscopy (E-TEM), we have studied the structure of self-assembled microparticlesand dodecanethiol-capped silver nanoparticles at Pickering emulsion interfaces.Monodisperse polystyrene particles were found to aggregate and form small patches atthe polydimethylsiloxane/water emulsion interface with local “hexagonal” order.Polystyrene particles with different sizes (1 μm and 4 μm) and, intriguingly, differentwettability (hydrophobic and hydrophilic) could simultaneously aggregate to theemulsion interface and form mixtures on it. In contrast, the dodecanethiol-capped silvernanoparticles (1-5 nm) formed randomly distributed multilayers at thetrichloroethylene/water interface with an inter-particle distance varying from closecontact to approximately 25 nm. Our work, to the best of our knowledge, offers the firstwork revealing the detailed self-assembled structure of nanoparticles at a liquid/liquidinterface when the size of the nanoparticles is comparable to the molecular dimensionof the liquids.