The optically active dopant distribution in a Tm(3+)-Yb(3+) doped silica based glass ceramic sample has been investigated. A systematic analysis of the upconversion fluorescence of the Tm(3+)-Yb(3+) codoped glass and glass ceramic has been performed at room temperature. Tm(3+) and Yb(3+) single doped glass and glass ceramics have also been included in the study. Upon infrared excitation at 790 nm into the (3)H(4) level of the Tm(3+) ions a blue upconversion emission is observed, which is drastically increased in the Yb(3+) 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, (3)H(4), (2)F(5/2), and (1)G(4) were interpreted in the glass ceramic samples. The contribution of the optically active Tm(3+) and Yb(3+) ions in the crystalline and in the vitreous phase of the glass ceramic was distinguished and the ratio of Tm(3+) ions in the crystalline phase could be quantified for the 1 mol % Tm(3+)-2.5 mol % Yb(3+) glass ceramic. A surprising result was obtained for that concentration: the main contribution to the upconversion emission of the glass ceramic is due to Tm(3+)-Yb(3+) ions in the vitreous phase.