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Gadolinium oxide films, prepared by the sol-gel process, present waveguiding properties. Their structures were studied by waveguide Raman spectroscopy (WRS) and confirmed using transmission electron microscopy (TEM). The structural evolution of the layers with annealing temperatures from 650 up to 900 • C was investigated. The WRS results and TEM observations were correlated and revealed that crystallization started at 650 • C into the cubic phase. The mean diameter of the crystallites dispersed in the amorphous phase is around 5 nm at this temperature. The film is totally crystallized into the cubic phase at 700 • C. The monoclinic phase simultaneously appears at 800 • C and both phases still co-exist after heat treatment at 900 • C. The films present a heterogeneous structure at 900 • C with grain sizes varying from 5 up to 50 nm and as consequence their waveguiding properties strongly decrease.