The synthesis of a series of new layered rare-earth hydroxide solid solutions and their transformation into (EuxGd1-x)(2)O-3 crystallites are described. Highly crystalline platelets of EuxGd1-x(OH)(2.5)Cl-0.5 center dot 0.9H(2)O solid solutions with various Eu3+/Gd3+ ratios were prepared through a homogeneous precipitation method. The hydroxide solid-solution samples exhibited characteristic Eu3+ photoluminescence properties through the energy transfer from Gd3+ to Eu3+ and the self-excitation of Eu3+. Cubic (EuxGd1-x)(2)O-3 crystallites were obtained via quasi-topotactic transformation of EuxGd1-x(OH)(2.5)Cl-0.5 center dot 0.9H(2)O solid solutions above 800 degrees C. The as-transformed cubic (EuxGd1-x)(2)O-3 crystallites well retained the original platelet morphology and single crystalline nature, and exhibited greatly enhanced photoluminescence properties with respect to the precursor hydroxides. The Eu3+ content of 0.05 in the cubic (EuxGd1-x)(2)O-3 gave a maximum luminescence intensity, which is comparable with that of a commercial Y2O3:Eu phosphor.