The crystal structures of CeAlO 3 and the solid solutions Ce 1Àx R x AlO 3 (R ¼ La, Nd), and their thermal behaviour in a wide temperature range of 12À1200 K have been precisely determined by means of in situ high-resolution X-ray powder diffraction technique applying synchrotron radiation, thermal analysis and magnetic measurements. The unique sequence of the reversible phase transitions I4/mcm2Imma2R3 ¯c2Pm3 ¯m has been detected in CeAlO 3 and solid solutions formed in the pseudo-binary system CeAlO 3 -LaAlO 3 . In the Ce 1Àx Nd x AlO 3 system, the samples with x ¼ 0.3 and 0.5 compositions undergo three phase transformations I2/m2Imma2R3 ¯c2Pm3 ¯m, whereas for the Ce-rich sample Ce 0.9 Nd 0.1 AlO 3 four successive transitions are observed: I4/mcm2 I2/m2Imma2R3 ¯c2Pm3 ¯m. Crystal structure parameters of all structural polymorphs of CeAlO 3 and solid solutions based on them as well as their thermal evolution are reported. Based on in situ powder diffraction and DTA/DSC data, the phase diagrams of the pseudobinary systems CeAlO 3 -LaAlO 3 and CeAlO 3 -NdAlO 3 are constructed together with a combined phase diagram, where the transition temperatures are presented as a function of the average radius of rare-earth cations.