Rare-earth-containing oxide nanocomposites are prepared in a controlled fashion from porous GaAs templates. The initial porous GaAs network is replaced by a β-Ga₂O₃ one during annealing at temperatures from 500 to 900 °C. The impregnation of Eu and Er lanthanides from EuCl ₃:C₂H₅OH and ErCl₃:C ₂H₅OH solutions results in the formation of xenotime EuAsO₄ and ErAsO₄ microcrystals finely dispersed into the native oxide matrix. The 4f-4f intrashell transitions in Eu³⁺ and Er³⁺ ions ensure red and green emission from EuAsO₄ and ErAsO₄ nanophases. These nanocomposites may prove useful in future generations of optoelectronic and photonic devices.