Abstract: The process for the preparation of an yttrium and at least one rare earth mixed oxide comprises the following steps: a. mixing a precursor of the yttrium and at least one rare earth mixed oxide with a flux comprising a barium halide and a boron compound; b. calcining the mixture of step a to obtain said mixed oxide. This mixed oxide may be used as a phosphor in the manufacture of coloured fluorescent lamps, cathode ray tubes and plasma display panels.