Composites of HDPE and glass or mica particles with different aspect ratios and surface modifications were prepared. The aspect ratio of the mica platelets was evaluated by image analysis of SEM micrographs. Test specimens, in which the platelet faces are oriented either parallel or perpendicular to the specimen flat surface, were prepared and the orientation of the particles was assessed by X-ray diffraction. Oscillatory rheological measurements in the linear viscoelastic regime were carried out. The in-plane and out-of-plane shear behavior was measured and the Halpin-Tsai structural parameter zeta calculated. For in-plane shear, zeta slightly decreased with increasing aspect ratio, whereas for out-of-plane shear, strongly increased. Surface treatment of the mica particles had practically no influence on the results in the investigated loading range (volume fraction less than or equal to7%). (C) 2004 Elsevier Ltd. All rights reserved.