Potassium hexacyanoferrate(III) transforms slowly to potassium hexacyanoferrate(II) when heated in air at 200°. At 250° and higher temperatures the crystalline phase consists mainly of potassium carbonate. Between 200 and 300° all the preparations are ferromagnetic. At 350° this property ceases slowly with time. It has been proved indirectly that the ferromagnetism is due to the ferromagnetic modification of iron(III) oxide (γ-Fe2O3).