Under repeated tensile strain, many particle-filled polymers such as silica-filled polydimethylsiloxane (PDMS), exhibit a reduction in stress after the initial extension, the so-called Mullins effect, and the mechanism(s) responsible for this is considered to be a major unsolved mystery of polymer physics. We report here the first observation of the absence of this effect in cross linked, silica-filled PDMS when the second strain axis is perpendicular to the initial strain axis. This result poses a challenge for existing theories. We propose a mechanism to account for the Mullins effect that is consistent with our experimental observations. (c) 2005 Published by Elsevier Ltd.