The evolution of the geometric and electronic structures within the entire series of lanthanide orthophosphate nanoparticles ͑ϳ2-ϳ 5 nm͒ has been determined experimentally with X-ray diffraction and near edge X-ray absorption fine structure spectroscopy. In particular, the interplay between electronic structure, crystal morphology, and crystal phase has been systematically studied. A missing local order in the crystal structure accompanied by multiple ion sites in the nanoparticles was revealed to be due to the small crystal size and large surface contribution. All lanthanide ions were found to be in "3+" configuration and accommodated in three different crystallization states: the larger lanthanide ions ͑La, Ce, Pr, Nd, Sm͒ in the monoclinic phase, the smaller ones ͑Er, Tm, Yb, Lu͒ in the tetragonal phase, and the intermediate lanthanide ions ͑Eu, Gd, Tb, Dy, Ho͒ in a "mixed phase" between monoclinic and tetragonal phases.

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