The paper summarizes the results of an experimental study of the formation of K–Ba high-Ti (and Cr) oxides synthesized in the chromite–rutile/ilmenite–K2CO3/BaCO3–H2O–CO2 systems at 1.8–5.0 GPa. Experiments confirm the conclusion that the formation of K–Ba high-Ti oxides characterizes the most advanced or repeated metasomatic stages in upper mantle peridotites, which lead first to the formation of simple Ti oxides and then to the formation of K–Ba high-Ti and Cr oxides. Relations between the oxides is a function of the activity of the K and Ba components in the fluid. The appearance of priderite corresponds to the highest activity of K in the mineral-forming media. Redledgeite is formed only in the Fe-poor chromite–rutile–H2O–CO2–BaCO3 system, and, in the system with ilmenite, minerals of the magnetoplumbite group preferably crystallize. A direct dependence of the Cr content in oxides on pressure is revealed. Raman spectra of K–Ba high-titanium oxides are presented. The structure of a potassium compound of a magnetoplumbite group with the chemical formula K0.90Ti5.16Cr2.94Fe2.54Mg0.87Al0.22Mn0.30O19 is studied by single-crystal X-ray diffraction using a synchrotron radiation. The obtained data can be used to specify the nomenclature of the magnetoplumbite mineral group.