Abstract: In the Na₂SO₄–K₂SO₄ system, nine phases are known. In this study, for the first time, the values of the thermal expansion coefficients of continuous high-temperature solid solutions crystallizing in the P6₃/mmc crystal system, low-temperature limited solid solutions based on natroaphthitalite and aphthitalite (P3-m1), and polymorph II Na₂SO₄ (Pbnm) in the Na₂SO₄–K₂SO₄ system are provided. The genetic succession of polymorphs P3m1 and P3-m1 ↔ P6₃/mmc, which manifested itself in the double sulfates of this system, is revealed.