Disclosed is a method for the preparation of particles of a rare earth ammonium double oxalate by mixing a first aqueous solution of ammonium oxalate and a second aqueous solution of a water-soluble rare earth salt. The double oxalate particles have a polyhedral particle configuration with a uniform particle size distribution, from which a rare earth oxide powder of a polyhedral particle configuration with good flowability can be obtained by calcination, when the first and second aqueous solutions are prepared under exact control of the concentrations of the respective ionic species and the aqueous slurry containing the precipitates of the double oxalate is subjected to an aging treatment during which changes take place in the particle morphology and in the crystallographic structure of the double oxalate particles.