The elongational flow-induced alignment process of a lamellar-forming polystyrene-b-poly(ethylene propylene) (SEP) diblock copolymer has been investigated by using elongational flow opto-rheometry (EFOR) and small angle X-ray scattering (SAXS). The lamellar domains of the compression molded SEP were aligned in a preferred direction by roll casting at 200 degrees C. In the EFOR measurement, the roll-cast film was uniaxially elongated at 180 degrees C at a Hencky strain rate, (epsilon) over dot of 0.01 s(-1) in the direction either perpendicular (denotes case I) or parallel (case II) to the direction of the lamellar normal. The apparent retardation, R-app, and initial moduli data measured during elongation suggest that either polystyrene (PS) or poly(ethylene propylene) (PEP) domain is preferentially elongated during the early stage of elongation. Transient elongational viscosity, eta((epsilon) over dot; t), measured during the case I elongation exhibits a continuous increase up to rupture with the absence of a morphological change. One measured during the case II elongation shows an initial increase up to 20 s, followed by a slight decrease by 80 s prior to a predominant increase before rupture. The SAXS patterns of the elongated samples show that the lamellar domains subjected in the case I elongation persist during elongation without any prominent structural change, whereas those subjected in the case II elongation are highly deformed and rotated by about 90 degrees, displaying a strong strain-induced hardening in the final stage of elongation. (C) 2008 Elsevier Ltd. All rights reserved.