Monomer droplet nucleation has been successfully tracked in a seeded semibatch miniemulsion polymerization with miniemulsion feeding. Taking advantage of the marker capacity of CeO2 nanoparticles two polymerizations were designed. In the first one the seed contained nanoceria while a blank monomer miniemulsion was fed to the reactor and in the second reaction the opposite was done; namely, the seed did not contain any CeO2 and the miniemulsion fed did. In this way, the presence of the CeO2 in either seed or nucleated particles allowed tracking the fraction of droplets that efficiently nucleated during miniemulsion feeding. Conventional TEM and cryo-TEM were used to monitor the nucleation efficiency by analyzing separately the particles that did or did not contain CeO2 nanoparticles.