Simultaneous sharp changes in the energy of К x-ray lines of Ce and Yb have been observed experimentally in Ce_{0 9 x} Yb_x Th₀,. These changes are reversible as the temperature is varied. The effect is shown to be due to a derealization of 4f electrons of Ce and Yb (the electronic transitions Ce³⁺, 4/^l-Ce⁴⁺, 4/°, Yb²⁺, 4/¹⁴Yb³⁺,4/¹³).