Several magnesium matrix composites were investigated by means of mechanical spectroscopy. A modulus anomaly has been observed in the composites reinforced with fibers while the phenomenon does not occur with particles. This effect is attributed to bonding and debonding of fibers to the matrix, which is due to thermal stress relaxation by motion of dislocations. The modulus anomaly depends strongly on the orientation of the glide plane of the dislocations with respect to the matrix fiber interface and on the aspect ratio of the fibers. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim