Single-phase M-type hexagonal ferrites Sr 1-xLa xFe 12O 19 (0≤ x≤1) were prepared by a ceramic route. The stability limits of the ferrite phases were determined with a combination of various microscopy techniques, electron-probe micro-analysis, powder X-ray diffraction and thermal analysis. SrFe 12O 19 ( x=0) is stable up to 1420 °C, whereas LaFe 12O 19 ( x=1) exists between 1360 and 1400 °C only. The lattice parameters of Sr 1-xLa xFe 12O 19 exhibit a linear variation with x, i.e. a0 slightly increases and c0 decreases with x, leading to a decrease of the unit cell volume with x. The saturation magnetization at T=5 K decreases with increasing La concentration. Room temperature Mössbauer analysis shows that the Fe 3+/Fe 2+ valence change occurs in the 2a sites for the whole composition range.