The cyclic deformation of two polyurethane elastomers that differed in soft segment content and molecular weight was investigated. The microphase-separated morphology of the polyurethane with higher soft segment content consisted of hard segment domains dispersed in a soft segment matrix. In the polyurethane with lower soft segment content, the hard segment domains appeared to be partially cocontinuous. Following an initial 'conditioning' cycle, both polyurethanes exhibited reversible elastomeric behavior. Structural changes that occurred during conditioning were investigated using atomic force microscopy and Fourier transform infrared dichroism. The results provided the basis of a structural model for the deformation behavior. Yielding and reorganization of hard domains resulted in a highly oriented microfibrous morphology. Subsequent unloading and reloading were associated with reversible relaxation and reformation of the microfibrous entities. The elastic behavior of the conditioned polyurethanes was satisfactorily described by classical rubber theory with inextensibility. The structural model proposed here extended previous efforts to describe the deformation processes Of polyurethanes during cyclic loading. (c) 2005 Elsevier Ltd. All rights reserved.