Abstract Nonstoichiometric hexaferrite particles containing both Ba and K, with typical composition K 0.8Ba 0.6Fe 10.5Co 0.25O 17, have been prepared by partial ion exchange in an aqueous solution of Ba(NO 3) 2at 40–80°C, from β″ ferrite precursor phase K 2Fe 10.5Co 0.25O 17. The β″ structure is maintained after the exchange reaction and the hexaferrite structure is formed by heating at T> 600°C. Exchange of K +by H 3O +also takes place in the aqueous solution, resulting in the formation of Fe 2O 3during annealing. The crystal structure is described as an M-type hexaferrite structure with intergrown β-type cells. Saturation magnetization in a field of 20 kOe at room temperature is 35–38 emu/g; coercive field H c≈ 2.0 kOe and T c= 460°C. Low magnetization values have been related to the low percentage (66%) of M-type cells in the structure.