Intense green upconversion (UC) emission has been obtained from Er-Yb-Mo tridoped ZrO2. The effect of different doping agents on the crystalline structure of the materials was investigated by XRD. Phase transformation from monoclinic to tetragonal ZrO2 occurred due to doping an appropriate amount of oversized trivalent cations and generating oxygen ion vacancies. Green UC emission in Er3+ doped ZrO2 and red UC emission in Er3+-Yb3+ codoped ZrO2 were observed upon 980 nm excitation. Remarkably, enhancement of the green UC emission as much as 20 times was achieved by tridoping with Er3+-Yb3+-Mo6+ ions in ZrO2. The high efficiency of the UC can be attributed to a novel energy transfer process in which Yb3+-MoO42- sensitized Er3+ ions at (IF7/2)-F-2, T-1(1)) excited state. (C( 2014 Elsevier B.V. All rights reserved.