Samples of general formula BaFe12-2xCo3xO19 (x=0.1, 0.3, 0.5) have been synthesized by ceramic as well as citrate gel routes. The X-ray diffraction patterns indicate that M-type phase forms for x=0.1, while W-type phase is favourable for x=0.3 and 0.5. The results tor x=0.3 are particularly interesting. While the X-ray diffraction data show evidence of a monophasic W-type hexaferrite, the magnetic, thermal and microstructural investigations indicate a secondary phase presumably at grain boundaries. Transformation of M- to W-type hexaferrite has been explained in terms of different possibilities such as nucleation of secondary phases, formation of vacancies and different site occupation of ions in the structure.