The effect of the mixing states of CaF₂-like phases appearing in the Zr-Ce-Pr-O system on their thermochemical behavior has been discussed on the basis of the data accumulated over the past few decades. In the Pr-O system, a series of CaF₂-like Pr_nO_2n-2: Pr³⁺₄Pr⁴⁺_mO_2(4+m)-2 oxides are well-known; the metastable phase (m = 4) may appear in a mixing state with the m = 5 or 3 phase for the Pr_nO_2n-2 thin film sputtered on a CaF₂-related Y₂O₃ substrate. In the Zr-Ce-O system, there exist many CaF₂-like and -related phases: tetragonal t, t', t'', cubic c, κ-Ce₂Zr₂O₈, and so on. In the Zr-Pr-O system, the novel CaF₂-type λ-(Zr_1/4Pr_3/4)O_2-y phase with large oxygen nonstoichiometry has been discovered. For the quasi-binary solid solutions, Zr_2/8Ce_(6-x/8)Pr_x/8O_2-y (0≤ x ≤6), the (c, t'') and λ phases appear in the Ce-rich and Pr-rich regions, respectively. The extremely resembling c, t'' and λ phases can form junctions with coherent interfaces, accompanied by introduction of oxygen vacancy around them, when the samples pre-fired at a sufficiently high temperature. The most excellent results for CH₄ steam reforming test have been obtained with Ni catalyst supported by the oxides possessing the junctions; the catalytic activity of NiO has been reported.