We have investigated the mechanism of the spontaneous growth of a gold nanoparticle film on a container wall when an aqueous dispersion of gold nanoparticles is shaken with an oil phase containing octadecylamine, as first described by Mayya and Sastry (Mayya, K. S.; Sastry, M. Langmuir 1999, 15, 1902.). Experimental evidence is described, which shows that the film growth is driven by the coalescence of particle-coated emulsion drops with the flat oil-water interface separating the oil and water phases.