The optically active dopant distribution in a Tm3+-Yb3+ doped silica based glass ceramic sample has been investigated. A systematic analysis of the upconversion fluorescence of the Tm3+-Yb3+ codoped glass and glass ceramic has been performed at room temperature. Tm3+ and Yb3+ single doped glass and glass ceramics have also been included in the study. Upon infrared excitation at 790 nm into the H-3(4) level of the Tm3+ ions a blue upconversion emission is observed, which is drastically increased in the Yb3+ codoped samples. A rate equation model confirmed the energy transfer upconversion mechanism. Based on these results, the temporal dynamic curves of the levels involved in the upconversion process, H-3(4), F-2(5/2), and (1)G(4) were interpreted in the glass ceramic samples. The contribution of the optically active Tm3+ and Yb3+ ions in the crystalline and in the vitreous phase of the glass ceramic was distinguished and the ratio of Tm3+ ions in the crystalline phase could be quantified for the 1 mol % Tm3+-2.5 mol % Yb3+ glass ceramic. A surprising result was obtained for that concentration: the main contribution to the upconversion emission of the glass ceramic is due to Tm3+-Yb3+ ions in the vitreous phase. (C) 2004 American Institute of Physics.