Neutron diffraction measurements have been used to track the microstructural changes which take place on milling SrFe12O19 and on subsequent annealing treatments. Mechanochemical treatment causes SrFe12O19 to transform progressively to disordered material, incorporating nanostructured and amorphous phases, with chemical reduction to haematite, alpha -Fe2O3 and SrO. Good agreement is obtained between the behaviour of the recrystallisation process as investigated by room temperature measurements of annealed products, and by separate in situ measurements at high temperatures. The recrystallisation process, which starts at similar to 640 degreesC and is complete by similar to 780 degreesC with mean particles sizes for SrFe12O19 of d similar to 130 nm at 800 degreesC, follows a similar behaviour to that of the spontaneous magnetisation and magnetic coercivity as reported previously for milled and annealed BaFe12O19 and SrFe12O19.