The effect of oxidizing annealing on the phase composition, microstructure, and magnetic properties of solid solutions based on Pb-containing strontium M hexaferrite is studied. The solid solutions prepared by different methods (ceramic route and glycol-citrate sol-gel process) are shown to differ in composition. Oxidizing annealing (air, 500–700°C) of single-phase materials leads to the formation of nanometer-sized (10–20 nm) equiaxed precipitates on some of the faces of the (initially smooth) hexaferrite grains. In some cases, such precipitates form chains running over the entire surface of the hexaferrite grain. Some of the annealed solid solutions contain a second phase, which has not been identified. Annealing slightly increases the coercive force of the solid solutions (by 2 to 25%, depending on their composition), which may be due to oxidation-induced second-phase precipitation or phase segregation of the solid solution within the hexaferrite grains.