The structure of the layered perovskite (\mathrm{Na}{2} \mathrm{Nd}{2} \mathrm{Ti}{3} \mathrm{O}{10}) was investigated by refining (\mathrm{X})-ray powder diffraction data using the Rietveld method. The (\mathrm{Nd}^{3+}) cation is located in the intraslab perovskite site and the (\mathrm{Na}^{+})cation in the interlayer space. The homologous compound (\mathrm{K}{2} \mathrm{Nd}{2} \mathrm{Ti}{3} \mathrm{O}{10}) spontaneously intercalates water, and acid exchange of this compound leads to the formation of (\mathrm{H}{2} \mathrm{Nd}{2} \mathrm{Ti}{3} \mathrm{O}{10}), (x \mathrm{H}_{2} \mathrm{O}). Prior to condensation at (900^{\circ} \mathrm{C}), the thermolysis of this solid acid yields an intermediate phase which, despite the total removal of water, retains the layered structure from 600 to (850^{\circ} \mathrm{C}). The thermal evolution of the protonated form has been studied by thermal analysis and crystallographic techniques and is found to exhibit two main steps. The first corresponds to the removal of water, which is complete at (600^{\circ} \mathrm{C}), and results in an intermediate phase containing fivefold-coordinated titanium cations and a statistical distribution of (\mathrm{Nd}^{3+}) in the available intra- and interslab sites. The second step can be considered as a complex condensation reaction leading to a 3D-cation defective perovskite. 1994 Academic Press, Inc.