β-Fe₂O₃ is a rare crystalline polymorph of the ferric oxide family with an interesting application potential, e.g., in photocatalysis. In this study, the effect of different alkali salts addition, namely NaCl and KCl, on the preparation of β-Fe₂O₃ via thermally induced solid-state reaction was investigated. Two series of samples were prepared by calcining two different mixtures, Fe₂(SO₄)₃ + NaCl (molar ratio 1:3) and Fe₂(SO₄)₃ + KCl (molar ratio 1:3) at temperatures from 350 to 700 °C. Although the addition of either alkali salt led the preparation of β-Fe₂O₃ particles in wide temperature range up to 650 °C, differences in the overall phase composition and β-Fe₂O₃ purity were observed between the two series. The addition of KCl to Fe₂(SO₄)₃ allowed the preparation of pure β-Fe₂O₃ (≥ 95%) in relatively wide temperature range of 450‒600 °C, while in the case of NaCl, pure β-Fe₂O₃ (≥ 95%) was found only in samples calcined at 500 °C and 550 °C. Other phases could be identified as additional ferric oxide polymorphs, γ-Fe₂O₃ and α-Fe₂O₃. The in situ XRD results suggest that, in the case of NaCl + Fe₂(SO₄)₃ reaction, simultaneous formation of β-Fe₂O₃ and α-Fe₂O₃ may be possible between 350 and 500 °C, depending on the reaction conditions.