Latex particles have been synthesized in two-steps emulsion polymerizations under starving conditions, and the internal structure of these particles has been investigated by fluorescence nonradiative energy transfer (NRET), in order to check for the formation of core-shell particles. The polymers were based on methyl methacrylate (MMA) and butyl methacrylate (BMA) as monomers. The energy donor monomer was introduced during the first step of the polymerization, and the energy acceptor monomer during the second step. The T-g of PBMA is too low, compared to the polymerization temperature T-p (T-p = 80 degrees C), to observe a separation between the donor- and acceptor-labeled PBMA chains inside the particles. On the contrary the T-g of PMMA is larger than T-p, and a separation between energy donor- and energy acceptor-labeled PMMA chains is observed with this polymer. The separation has the structure of a diffuse interface between the two labeled PMMA polymers. The study of other particles shows that the apparent fraction of mixing, f’(A), between donor- and acceptor-labeled polymer chains inside the particle decreases, as expected, as the incompatibility and the T-g of the polymers increase. Addition of a cross-linking agent during the first step of the polymerization leads to a decrease of f’(A), even in the case of the PBMA latex particles. Annealing of latex films and dispersions at temperatures above the T-g of the polymers allowed phase separation or mixing inside latex particles to be observed.