Upconversion (UC) phosphors comprising Er3+/Yb3+ co-doped rare-earth complex oxides, RExTayOz and RExNbyOz (RE = Y, La, or Gd), were synthesized via the gelable citrate complex method, and their UC emission properties were compared. This led to the discovery of an intense green-emitting UC phosphor, YTa7O19:Er3+/Yb3+, which is the first example of a YTa7O19 UC phosphor host material. Green UC emission obtained using optimum Er3+ and Yb3+ content was 20 times stronger than that obtained using a Y2O3 host.