High-surface-area zirconias are widely used as catalytic support of noble metals or oxygen sensor electrolytes in automobile exhaust-emission-control systems. Doping zirconia with small amounts of rare-earth (RE) elements may tailor its properties for better catalytic performance. The microstructure in terms of primary-particle size, surface area, porosity, and fractal aggregates of 10 mol% RE-doped zirconias (RE = Nd and Ce) and pure ZrO2 were characterized by nitrogen adsorption isotherm and small-angle neutron scattering measurements. The crystal phases of these powders were examined by neutron powder diffraction method. Fresh pure zirconia prepared by a hydrolysis method at low temperature consists of small (similar to 4 nm) particles and micropores. Subsequent heat treatments induce a transformation from microporosity to mesoporosity thereby an increase of particle size and a reduction of surface area. In the case of pure ZrO2 a crossover from a mass-fractal aggregate of rough particles to clustering of smooth particles at a heat-treatment temperature of 600 degrees C was observed. The Nd-modified zirconias prepared by a coprecipitation method, on the other hand, show high resistance to sintering and retain a small particle size and the mass-fractal aggregate after heat treatment at 600 degrees C. Because of the different oxidation states and ionic sizes of Nd3+ and Ce4+ ions, Nd0.1Zr0.9O1.95 and Ce0.1Zr0.9O2 powders exhibit different crystal phases, particle size distributions, and pore structure.