Phosphor particles (ca. 60 nm in size) of yttrium oxide co-doped with Er3+ and Yb3+ ions were prepared by a precipitation method in the presence of EDTA. Their physical properties were compared to much larger particles (ca. 600-800 nm in size) prepared in the absence of EDTA. All of the particles were shown to have crystallized in the cubic phase, and all exhibited blue, blue-green, and green-yellow upconversion emission when excited with laser light of wavelength equal to 632.8 nm. These upconversion emissions were shown to be excited by a two-photon process. The most intense yellow-green upconversion emission occurs when the crystallite size is between 75 and 200 nm and the particle size is 600 to 800 nm, Cross-relaxation processes between Er3+ ions are suggested to be responsible for the more efficient upconversion in the larger pal-tides. There is evidence from spectra taken in the temperature range 30 to -190 degreesC that there are two different hot bands in the given region of the spectrum. These two emission manifolds are explained as arising from the two different Er3+ lattice sites in the cubic Y2O3:Er3+ structure. When using red excitation (rather than infrared excitation), the presence of Yb3+ was found to be detrimental, as it diminished the upconversion intensity.