Aluminosilicate planar waveguides have been prepared using a sol–gel process and the dip-coating technique. The structure of Er3+- and Ce3+-doped as well as undoped films has been investigated as a function of the annealing temperature. Low-wavenumber Raman scattering and opto-geometrical properties of these thin films have demonstrated the role of Er3+ and Ce3+ ions in the densification and nucleation processes. Indeed, doping with rare-earth ions first hinders the densification process in the amorphous phase, while crystallization begins at a lower annealing temperature for the doped waveguides. It was found that the two behaviors are intimately linked to the doping effect.