Room-temperature blue and violet upconversion (UC) emissions of Er3+ at 409 nm (H-2(9/2) -> I-4(15/2)) and 390 nm ((4)G(11/2) -> I-4(15/2)) were observed in Y2O3:Yb3+-Er3+ nanocrystals by diode laser excitation of 980 nm. At low Yb3+ concentration both wavelengths result from three-photon UC processes. Unexpectedly, a four-photon and a decreased three-photon UC processes were observed at high Yb3+ concentration for the violet and the blue emissions, respectively. The four-photon process supposably results from the cross relaxation D-4(7/2) + I-4(15/2) -> (4)G(11/2) + I-4(13/2) of Er3+ ions with the D-4(7/2) state populated by the 2(2)F(5/2)(Yb) + F-4(9/2)(Er) -> 2(2)F(7/2)(Yb) + D-4(7/2)(Er) process, whereas the decreased photon process arises from the larger UC rate at high Yb3+ concentration. (c) 2007 Elsevier B.V. All rights reserved.