The internal structure of core-shell latex particles, containing a fluoropolymer in the shell, was determined by the fluorescence nonradiative direct energy transfer (NRET) technique. Contrast was achieved by labeling the core and the shell with fluorescent dyes, a donor and an acceptor, respectively and inversely. Donor fluorescence emission intensity was recorded in dilute latex dispersions. Analysis of donor fluorescence decay was carried out by choosing a donor concentration profile within the particles. The analysis revealed the absence of a sharp boundary between the core and the shell, but rather a diffuse interface, which was quantified to within a few tens of nanometers for our ca. 230 nm diameter particles. The scale of this interface was changed by varying the compatibility between the core and the shell.