Solid solutions between several lanthanide-containing dititanate and dizirconate compounds have been synthesized and identified using X-ray diffraction (XRD). Complete solubility was found in the pyrochlore–pyrochlore-type systems Er 2 Ti 2 O 7 –Nd 2 Zr 2 O 7 and Nd 1.2 Er 0.8 Ti 2 O 7 –Nd 2 Zr 2 O 7 . Some solubility of Nd 2 Ti 2 O 7 , a monoclinic dititanate, in defect fluorite Er 2 Zr 2 O 7 was observed, and it was determined that there is some solubility of Nd 2 Ti 2 O 7 in pyrochlore-type Nd 2 Zr 2 O 7 . The systems Nd 2 Zr 2 O 7 –TiO 2 and Nd 2 Ti 2 O 7 –ZrO 2 also were investigated to help define a region of pyrochlore solid solution formation in the system NdO 1.5 -TiO 2 -ZrO 2 . Additionally, attempts were made to synthesize pyrochlore-type solid solutions between Ce 2 Ti 2 O 7 and Ce 2 Zr 2 O 7 and to substitute titanium with zirconium in the perovskite-type phases SrNd 2 Ti 4 O 12 and Sr 2 Ce 2 Ti 5 O 16 .