The dielectric properties of thermally treated manganese doped Pb(Fe2/3W1/3)O-3 (PFW) ceramics were investigated. Modifications of the dielectric properties by thermal annealing were observed. Air annealing causes a sharpening of the dielectric permittivity and an increase of the permittivity maximum values, more pronounced as the amount of manganese dopant increases. Oxygen annealing does not cause evident variations in the dielectric permittivity curve. The presence of oxygen vacancies is evinced by thermal gravimetric measurements. The role of oxygen vacancies, as well as the associations of manganese ions with oxygen vacancies, on the annealing process, is proposed and discussed.