Powder X-ray diffraction (XRD) and Raman spectroscopic studies on Ce1-xYbxO2-x/2 and Ce1-xTmxO2-x/2 (0.0 <= x <= 1.0) are being reported in this manuscript. It has been observed that ceria when heavily doped with ytterbia and thulia get transformed from single-phasic F type to single-phasic C type through the formation of a biphasic mixture. The critical concentrations at which the phase transition takes place have also been ascertained from XRD measurements and verified using Raman spectroscopy. From XRD studies, it has been found that in the case of Ce1-xYbxO2-x/2, pure F-type lattice continues until x=0.4 and then the biphasic region comprising F-type and C-type solid solutions starts, which is followed by single-phasic pure C-type region above x=0.9. For Ce1-xTmxO2-x/2, XRD studies reveal that the biphasic region comprising C-type and F-type lattice is formed from x=0.5 onward. However, it has been confirmed from Raman spectroscopic studies that the C-type lattice is formed even in the sample at x=0.4. The discrepancy is believed to have originated because of the small domain size (smaller than the coherence length of X-rays) of the C-type lattice that are inhomogeneously distributed within the F-type solid solution in the composition Ce1-xTmxO2-x/2 (x=0.4), which could not be directly detected from XRD measurements.