To obtain a better understanding of how lanthanide-encoded polystyrene microbeads are formed during their synthesis, we carried out kinetic studies of two-stage dispersion copolymerization of styrene and acrylic acid (AA, 2 wt % based on styrene), with or without added TmCl3, in ethanol in the presence of poly(N-vinyl-2-pyrrolidone) (PVP) as a polymeric stabilizer. Particles of narrow size distribution and micrometer diameters were obtained, allowing us to compare rates of monomer conversion, Tm ion incorporation, and growth in particle size. In these reactions, AA or (AA + TmCl3) were added in a second stage (t = 1 h). Much of the AA incorporation into the particles occurred in the latter half of the reaction. In the presence of small amounts of TmCl3 there was some retardation of the polymerization reaction. The most striking result was that at the higher levels of TmCl3 most of the metal ion incorporation occurred late in the reaction.