The complete series of all nonradioactive lanthanide phosphates has been prepared in organic solution as ligand-capped nanoparticles. In all cases, well-dispersed particles with mean sizes below 10 nm are obtained. Despite the similar chemical properties of the lanthanides, the growth, the crystal structure, and the mean size of the nanocrystals are found to be strongly affected by the lanthanide ion employed. Very small nanoparticles are obtained for lanthanides for which the lattice energies of the bulk tetragonal xenotime phase and the bulk monoclinic monazite phase are similar. These smaller nanoparticles do not show the expected. phase transition from monoclinic to tetragonal and seem to have their own unique crystal structure. Possible explanations for these observations are discussed. Finally, we present the first results on the IR emission of Er3+-doped YbPO4 and (Lu, Yb)PO4 nanoparticles in solution.