The effect of Yb3+ ions on the blue upconversion (UPC) emission of Yb3+-TM3+ co-doped ZrO2 nanocrystals is reported. The blue (490 nm) UPC emission is due to (1)G(4-)(3)H(6) transition of Tm3+ and the pump power dependence of this UPC emission band are cubic indicating that three excitation photons are involved in the UPC process. An additional UPC emission band at 505 nm is observed when the Yb3+ concentration is above 1.25 mol%. The pump power dependence of the emission at 505 nm is quadratic, indicating that this emission may have its origin from excited Yb3+ pairs producing cooperative UPC. (c) 2005 Elsevier B.V. All rights reserved.