We study experimentally the energy of adsorption, Delta E, of nanoparticles and microparticles at the oil-water interface by monitoring the decrease of interfacial tension as the particles bind. For citrate-stabilized gold nanoparticles assembling on a droplet of octafluoropentyl acrylate, we find Delta E = -5.1 k(B)T for particle radius R = 2.5 nm and Delta E proportional to R-2 for larger sizes. Gold nanoparticles with (1-mercaptoundec-11-yl)tetra(ethylene glycol) ligand have a much larger binding energy (Delta E = -60.4 k(B)T) and an energy barrier against adsorption. For polystyrene spheres with R = 1.05 mu m, we find Delta E = 0.9 x 10(6) k(B)T. We also find that the binding energy depends on the composition of the oil phase and can be tuned by the salt concentration of the nanoparticle suspension. These results will be useful for controlling the assembly of nanoparticles at liquid interfaces, and the method reported here should be broadly useful for quantitative measurements of binding energy.