The time-dependent study at mild hydrothermal conditions revealed the kinetic relation between structurally unrelated bismuth ferrites. We report the synthetic conditions that disclose the in situ transformation of Bi25FeO39 into BiFeO3 by only extending the time for crystallization. We also demonstrate how a careful investigation of the early stages of multiferroics crystallization provides cheap and easy room-temperature access to the important sillenite-type of structure of Bi25FeO39. The crystal structure analysis by the Rietveld method and magnetization measurements provided detailed structure and magnetic relations between the synthesis conditions and the properties of the run product. The BiFeO3 antiferromagnetic behavior with a small ferromagnetic component arising from the particle's surface uncompensated spins is ascertained.