The effect of Gd substitution in M-type strontium hexaferrites has been examined in two series of samples, (Sr1-xGdx)O center dot 5.25 Fe2O3 and Sr1-xGdxFe12-xCoxO19, both prepared by the ceramic method, where x=0-0.40. The samples have been characterized by XRD, VSM and SEM-EDAX techniques. All substituted samples present primarily the hexaferrite structure. Sample (Sr0.95Gdx0.05)O center dot 5.25 Fe2O3 is single phase. Formation of impurity phases is affected by stoichiometry and presence of Co. In Sr-Gd samples, coercivity showed a maximum value of 305 kA/ m (3.8 kOe) for x=0.20, while remanence and saturation magnetization did not decrease. Coercivity and magnetization in the Sr-Gd-Co series decreased steadily with substitution degree. (C) 2007 Elsevier B.V. All rights reserved.