Both Er3+-Yb3+ and Er3+-Y3+ co-doped TiO2 films were prepared on fused silica by sol-gel processes. The effect of doping concentration upon the up-conversion fluorescence of Yb3+-Er3+ and Er3+-Y3+ co-doped TiO2 systems will be focused in this investigation. Although the influence of Yb3+ and Y3+ ions on the structural of Er3+-TiO2 host material is almost the same, different up-conversion emissions are observed. Both Er3+-doped and Er3+-Y3+ co-doped TiO2 films exhibit the up-conversion emission with green and red light. However, much stronger intensity of red light relative to green light is observed for the Er3+-Y3+ co-doped TiO2 system. For Er3+ (5 mol%)-Y3+ co-doped TiO2 samples, the relative intensity ratio (I-R/I-G) rapidly decreases with the increase of Y3+ concentration above 10 mol%. On the contrary, the relative intensity ratio (I-R/I-G) increases with increasing Yb3+ concentration for Er3+(5 mol%)-Yb3+ co-doped TiO2 samples. The up-conversion emission in the Er3+-Y3+ co-doped TiO2 system belongs to a two-photon absorption up-conversion process. However, the overall up-conversion efficiency in the Er3+-Yb3+ co-doped TiO2 system could be contributed from the complicated mechanisms. (C) 2003 Elsevier Science B.V. All rights reserved.