The phase relations in the NaB₃O₅-KB ₃O₅ pseudobinary system are investigated using high-temperature X-ray powder diffraction, differential scanning calorimetry, and the annealing and quenching techniques. Solid solutions Na _1-xK_xB₃O₅ (0.5 < x ≤ 1.0) based on the KB₃O₅ phase are found in the system at temperatures below 650-670°C. The replacement of potassium atoms by sodium atoms is accompanied by the contraction of the structure along the a and c axes. The parameter b (the direction of channels in the structure) is insensitive to the substitution of sodium atoms for potassium atoms. No isomorphic miscibility on the basis of NaB₃O₅ is revealed in the system. In the region from 0 to 50 mol % KB₃O₅, there exist two phases, namely, Na_1-xK_xB₃O₅ (x≈0.5) and β-NaB₃O₅.