Rare-earth substituted M-type barium hexaferrites with nominal formula (Ba1-xREx)O center dot 5.25(Fe2O3), with RE=Dy or Gd (x=0-0.20), were prepared by the chemical coprecipitation method. Complex permittivity epsilon(r)* and permeability mu(r)* of HDPE-ferrite composites were measured with the coaxialline method in the 2-18 GHz frequency range. The electromagnetic behavior of the composites was correlated to dielectric polarization and magnetic interactions mechanisms, on the basis of the ferrite's physical and structural characteristics. The results show dissimilar effects on the microwave properties of the composites, that may be ascribed to different structural and physical configurations of the Dy- and Gd- substituted hexaferrites. (C) 2009 Elsevier B.V. All rights reserved.