The transformation of the crystal structure by isomorphous K ⁺-Cs⁺ substitutions in the non-tetrahedral positions leading to the phase transition has been studied in the series of K _1-x Cs _x BSi₂O₆ solid solutions. The samples have been crystallized from the glass at 850°C for 10 h. According to the data on the crystal structures refined by the Rietveld method, the compositions with 0 ≤ x ≤ 0.35 crystallize in space group -Rfis1-temp̄43d, structural type KBSi₂O₆, and those with 0.37 ≤ x ≤ 1.0, crystallize in Ī3d, structural type CsBSi ₂O₆. The reversible cubic-cubic phase transition Ī43d Ī3d occurs in the composition range x = 0.35-0.37.