The thermal behavior of Na₃H(SO₄)₂ is studied in the temperature range –180..220°C in a pre-vacuum. At 180°C, the compound decomposed into a mixture of phases: Na₂SO₄ (Cmcm), Na₂SO₄ (P6₃/mmc), Na₂SO₄ (Fddd), and Na₂S₂O₇. The maximal thermal expansion was observed near the α₃₃ axis. The anisotropy of thermal expansion sharply decreased together with the increase in temperature.