The influence of styrene-ethylene-butylene-styrene (SEBS) triblock copolymers on the interfacial interactions in heterogeneous polystyrene/low-density polyethylene (PS/LDPE) blends was studied by simultaneous Fourier transform infra-red (FTi.r.) spectroscopic and stress-strain (rheo-optical) measurements. Values of the dichroic ratio (DR) for a specific absorption band of the dispersed LDPE phase were determined from the polarized spectra as a function of extension (lambda), and were taken as a measure of the molecular orientation of the LDPE. A linear relationship between DR and In lambda was found, and the slope was interpreted as a reflection of the interfacial interactions in the heterogeneous blends. The slope increased with the amount of a low-molecular-mass SEES which was added, but was less affected by the addition of a high-molecular-mass SEES. The difference was explained by a lower concentration of the high-molecular-mass copolymer at the interphase.