The preparation and characterization are described of a series of core-shell poly(butyl methacrylate) latex particles containing varying amounts of carboxylic acid groups. Matched pairs of particles, labelled with either donor (phenanthrene) or acceptor (anthracene) groups, were prepared by three-stage emulsion polymerization under monomer-starved conditions in which varying amounts of methacrylic acid were introduced into the third-stage polymerization. Films were prepared from the latex. The rate of polymer interdiffusion in the films, as followed by energy transfer measurements, decreased as the carboxyl content of the latex shell increased.