Highly fluorescent conjugated polymer nanoparticles were prepared directly by polymerization in aqueous miniemulsion, employing Glaser coupling polymerization as a suitable step-growth reaction. A 4,4'-dinonyl-2,2'-bipyridine-modified catalyst was found to be suited for the polymerization in the aqueous heterophase system. Nanoparticles of poly(arylene diethynylenes) (arylene = 2,5-dialkyoxy phenylenes and 9,9'-dihexyl fluorene) with molecular weights in the range of M(n) 10(4) to 10(5) g mol(-1) and with sizes of < or = 30 nm, as observed by TEM, result. N,N'-di(4-ethynylphenyl)-1,7-di[4-(1,1,3,3-tetramethylbutyl)phenoxy]perylene-3,4:9,10-tetracarboxdiimide or 2,7-diethynylfluorenone was converted completely during the heterophase polymerization to afford colloidally stable nanoparticles of poly(arylene diethynylenes) with 0.1-2 mol % covalently incorporated perylene dye and 2-9 mol % of covalently incorporated fluorenone dye, respectively. Fluorescence spectroscopy of the aqueous dispersions reveals an efficient energy transfer to the dye in the nanoparticles, which enables a variation of the luminescence emission color between red (lambda(em) (max.) ca. 650 nm) and the green emission of the nanoparticles without dye.