Aqueous suspensions of single-wall carbon nanotubes (SWNTs) prepared by traditional methods from the supernatant of ultracentrifuged micellar SWNT mixtures are shown to be strongly heterogeneous with respect to the quantum yield of their constituents. The heterogeneities are isolated using preparative ultracentrifugation in iodixanol density gradients and are optically characterized by photoluminescence and linear absorption spectroscopy. We find that the most buoyant fractions with a density of 1.055 +/- 0.005 g center dot cm(-3) have the highest photoluminescence quantum yield eta of 1.1%, a factor of 5 higher than that of the supernatant. Denser fractions with lower eta make up for about (2)/(3) of these samples and contain mostly small ropes in which the quantum yield is reduced by nonradiative decay through coupling to metallic tubes.