Simultaneous measurements of microscopic infrared dichroism, mesoscale deformation, and macroscopic stress have been made for a microphase-separated film of poly(ether-block-amide) 4033 during uniaxial stretching at temperatures between 30 and 91 degrees C, well below the melting point of the hard polyamide-12 (PA) domains. Before the onset of dramatic microstructural alterations, the true stress-strain relationship on the mesoscale can be described with an interpenetrating network model, and poly(tetramethylene oxide) (PTMO) soft segments undergo affine deformation. Beyond a threshold strain at which stress from the soft network becomes larger than that from the hard network, plastic deformation occurs in the hard PA domains, and this is accompanied by the downward derivations of the true stress and molecular orientation of PTMO blocks from the model predictions. (c) 2005 Wiley Periodicals, Inc.