A series of Gd-substituted Ba hexaferrites with nominal formula (Ba1-xGdx)O center dot 5.25 Fe2O3 (x=0-0.30) were prepared by the chemical coprecipitation method from nitrate precursors and heating at T=800-1200 degrees C for 2 h. The samples have been examined by x-ray diffraction, vibrating-sample magnetometer, and scanning electron microscopy methods. Gd substituted samples form single phase materials with the M-type hexaferrite structure at all heating temperatures, in the range of x <=0.10-0.20. The saturation magnetization (at 1.8 T) varies slightly with x in most cases and, for x=0.05-0.10, it increases up to 66.7 A m(2)/kg, exceeding the value of the unsubstituted hexaferrite. A strong enhancement of the coercivity is observed for all substituted samples, with maximum values H-c=457 kA/m for the single-phase x=0.10 sample annealed at 1000 degrees C and H-c=477 kA/m for the x=0.25 sample annealed at 1100 degrees C which contains Fe2O3 and GdFeO3 impurities. As the variation of coercivity with either substitution rate (x) or annealing temperature is not monotonic, three different factors may account for the high coercivities that are obtained: (a) an inhibition of grain growth due to the presence of Gd, (b) a possible inherent effect on magnetocrystalline anisotropy, especially for single phase samples, and (c) a microstructural effect of secondary phases. (C) 2008 American Institute of Physics.