Elongational how behaviour of a poly(styrene-co-acrylonitrile) (SAN)/poly(methyl methacrylate) (PMMA) blend of a near critical composition (similar to 40/60) was examined below and above its lower critical solution temperature T-s(= 195 degrees C) via elongational flow opto-rheometry (EFOR) which enabled us to simultaneously monitor transient tensile stress sigma(epsilon over dot (0); t) and birefringence Delta n (epsilon over dot (0); t) as a function of time t. Thus the stress optical rule (SOR) was tested on the blend below and above T-s. The morphology change with time t was examined by ex situ transmission electron microscopy (TEM) and the resulting TEM pictures were subjected to a fast Fourier transform (FFT) analysis. In the miscible region at 150 degrees C, the stress optical coefficient of the blend C-Blend [= Delta n(epsilon over dot (0); t)/sigma(epsilon over dot (0); t)] was found to be constant (congruent to -1.2 x 10(-9) Pa-1) independent either of the strain rate Co the Hencky strain epsilon(= epsilon over dot t) or sigma(epsilon over dot (0); t). At 215 degrees C above T-s, however, the sign of C-blend changed from negative to positive in the early stage of elongation and the value itself increased as the blend was stretched to a larger extent. Under such a condition the SOR was clearly violated, for which a large contribution of form birefringence was responsible due to the highly oriented and elongated domain structure, as revealed by TEM-FFT analysis, of the SAN phase with a large optical anisotropy. (C) 2000 Elsevier Science Ltd. All rights reserved.