Nanoparticles of synthetic Laponite clay can be coated with individual or mixed cyanine dyes to form J-aggregated assemblies in water. The absorption and fluorescence characteristics of these J-aggregates are substantially different from those of J-aggregates formed in crystals or colloids, suggesting that smaller J-aggregate domains are present on the disk-shaped nanoparticles. The mixed cyanine/Laponite aggregates exhibit similar but nonidentical absorption and fluorescence as compared to the individual dye aggregates, suggesting a mixed electronic structure. The fluorescence from the nanoparticle-adsorbed cyanines is subject to superquenching by a variety of electron- and energy-accepting quenchers. These quenchers can be introduced by coadsorption with donors on the clay (mixed aggregates) or by addition of acceptors to preformed donor aggregates in aqueous suspensions. For some of the energy-accepting cyanines, it is found that strong sensitization of fluorescence from a (mostly) acceptor state is observed, even in cases where the monomer dye is non- or very weakly emissive. In limiting cases, 50% quenching is observed at levels of 1 quencher per similar to400 monomers or 4 quenchers per nanoparticle.