Sonogashira coupling of dibromo- and diethynyl-substituted benzenes and fluorenes in aqueous mini-emulsion afforded colloidally stable dispersions of highly fluorescent 60-120 nm particles of poly(arylene ethynylene)s of molecular weights M(n) 4 × 10(4)-2 × 10(5) g mol(-1) with solids contents up to 15 wt %. By covalent incorporation of diethynyl pyrrolo-pyrrole or diethynyl fluorenone in the mini-emulsion polymerization the emission color of these photostable nanoparticles can be tuned from blue (λ(em max) 470) to orange at a given excitation wavelength due to an efficient energy transfer to these acceptors. Two-photon action cross sections of the particles amount to 10(6) to 10(7) GM. The particles from emulsion polymerization are taken up by HeLa cells without an adverse effect on the cells. Differentiation of the location in cells of particle species varying in emission color was demonstrated for both linear and two-photon excitation microscopy in the NIR regime.