Lanthanide-modified CdSe quantum dots (CdSe(Ln) QDs) have been prepared by heating a solution of Cd(oleate)(2), SeO2, and Ln(bipy)(S2CNEt2)(3) (bipy = 2,2'-bipyridine) to 180-190 degrees C for 10-15 min. The elemental compositions of the resulting CdSe(Ln) cores and CdSe(Ln)/ZnS core/shell QDs show this route to be highly reproducible. The optical absorption spectra of these composite materials are similar to those of the unmodified nanocrystals, but the QD-centered band edge photoluminescence (PL) is partially quenched. The time-gated emission and excitation spectra of the CdSe(Ln) cores display sensitized lanthanide-centered PL upon higher energy excitation of the nanocrystal host but not upon excitation at the lowest energy QD absorption band. Growth of the ZnS shell led to the depletion of about 60% of the lanthanide ions present together with depletion of nearly all of the lanthanide-centered PL. On these bases, we conclude that the lanthanide-centered PL from the CdSe(Ln) cores originates with Ln(3+)-related trap states associated with the QD surface.