A closed porous structure in ceramic materials was investigated by spin-echo small-angle neutron scattering. A series of oxygen-conductive ceramic samples based on bismuth molybdate having general formula Bi12.8X0.2Mo5O34 ± δ (X = Mg, Ba, Ca, Sr) was obtained by powder sintering for 6−45 h at a temperature near the melting point. A sample validation made by means of scanning electron microscopy and X-ray fluorescence analysis showed that the chemical composition was stoichiometric, the samples were single-phase and contained clean pores between crystal grains. The pore size was determined by spin-echo small-angle neutron scattering and ranged from 2.2 to 3.5 μm. The long time of sintering was shown to result in increase of the pore size (mean diameter rise is up to 30%). The fractal pore structure in the considered materials was absent.