Abstract This paper reports the phase distribution and particle size of Eu 3+:Y 2O 3 nanoparticles prepared by gas-phase condensation as a function of preparation conditions and annealing. Annealing improves the crystallinity of the as-prepared nanoparticles but also increases the particle size. Gas-phase condensation at a low pressure (10 Torr) produces 5-nm Eu 3+:Y 2O 3 in multiple phases. Annealing the 5-nm nanoparticles produces single-phase material in the cubic structure with a doubling of the particle size. Higher chamber pressures produce larger particles. A pressure of 400 Torr produces 13-nm Eu 3+:Y 2O 3 in the monoclinic structure. Annealing the 13-nm particles produces a mixture of monoclinic and cubic phase material with a 50% increase in average particle size. We also report preliminary attempts to disperse Eu 3+:Y 2O 3 and Eu 2O 3 nanoparticles in different surfactant and polymer solutions.