The solid solution technique is one of the oldest but most powerful methods in the search for advanced materials. Hume-Rothery rules regarding solid solutions are well established for alloys, but little is known about oxides. In this study, we focused on the spinel oxides of Fe[Li1/2Fe3/2]O4−LiFeTiO4−Li[Li1/3Ti5/3]O4 to clarify their cation distribution, which necessitates other rules for substitutionally ordered/disordered solid solutions. Here, the chemical formula of the spinels is represented as Li1/2+x/2Fe5/2−3x/2TixO4 with 0≤x≤5/3. Synchrotron radiation X-ray diffraction measurements indicated two types of 1:3 cation order at the octahedral site over the wide x range; i.e., the cation order between one Li+ ion and three Fe3+ ions at 0≤x≤1 and the cation order between one Li+ ion and three Ti4+ ions at 1<x≤1.55. Rietveld refinements indicated that the formation of the cation order at 1<x≤1.55 is achieved by a combination of 1×(Li1−x1−y1+Fex13+Tiy14+) and 3×(Li1−x2−y2+Fex23+Tiy24+). Because of the mixed nature of the superlattice and the solid solution, Raman spectra at 1<x≤1.55 were similar to those with the disordered structure at x≥1.6. Details of the crystal structure, such as occupancies of the three cations and bond length, are discussed.