Nanoparticles of LaF3 doped with Ln3+ (Ln = Eu, Er, Nd, and Ho) have been prepared that are dispersible in organic solvents. From the spectrum of Eu3+ it has been concluded that the dopant ion occupies a La3+ site. The luminescence decays are fitted biexponentially. A given possible explanation for this is a different probability of nonradiative decay for ions at or near the surface and ions in the core of the particles. The lifetimes of Eu3+ luminescence at a doping concentration of 5% were 7.7 ± 0.2 and 2.9 ± 0.2 ms, indicative of very high quantum yields. The particles doped with Er3+, Nd3+, and Ho3+ are promising materials for polymer-based optical components, because they show luminescence in the telecommunication window (i.e., Er3+ at 1530 nm, Nd3+ at 1330 nm, and Ho3+ at 1450 nm).