W-type SrFe22+Fe163+O27 ferrite is a member of the hexagonal ferrite family and a potential permanent magnet material. However, its synthesis conditions are not fully understood yet. Samples were sintered either at 1400 degrees C in air and quenched, or at 1300 degrees C at reduced oxygen partial pressure. The precise stability conditions of this W-type ferrite were investigated in the temperature range of 1200 degrees C-1400 degrees C using thermogravimetry, XRD, and electron microscopy. At 1300 degrees C, the ferrite is stable at oxygen partial pressures of -3.1 <= logpO2 <=-1.6. At more oxidizing conditions, the ferrite decomposes into M-type ferrite and hematite, while at more reducing atmospheres Sr4Fe6O13 and magnetite are formed. The nonstoichiometry delta of SrFe18-delta O27 was derived from thermal analysis data at 1300 degrees C as function of oxygen partial pressure and was found to be mainly due to cation vacancies. Magnetization measurements show that this W-type ferrite exhibits M-s = 103 emu/g at T = 4 K, which agrees well with a ferrimagnetic spin arrangement according to Gorter's model. As alternative, Zn-substituted W-ferrite was found to be stable in air at 1200 degrees C with a large M-s = 123 emu/g at 4 K.