Abstract When solutions containing Fe(II), Ni(II), and SO42− ions were oxidized by a mixed gas of air and nitrogen at pH 7.1 (25 °C), a Ni(II)-bearing green rust II (GR-II), which is a uniform crystalline precipitate with a definite chemical composition, was formed. The Ni(II) content in the Ni(II)-bearing GR-II increased with an increase in the Ni(II) concentration in the reaction solution. The Ni(II)-bearing GR-II was spontaneously transformed into Ni(II)-bearing ferrite under a nitrogen atmosphere without any oxidant. The metal mole ratio of [Ni(II)/ Fe(III)]fer in the ferrite increased linearly with an increase in that of [Ni(II)/Fe(III)]grn in the Ni(II)-bearing GR-II (slope=0.193). From this relationship, the spontaneous transformation reaction was described as 2(Ni2+)x(Fe2+)2−x(Fe3+)(SO42−)(OH−)5→Ni0.39xFe3−0.39xO4+(2−1.61x)Fe2++1.61xNi2++Fe(OH)2+2SO42−+4H2O. This equation shows that Fe(II) ion in the GR-II is more readily incorporated into the ferrites than Ni(II) ion.