Linear conservative dichroism has been investigated as a potential tool for tracking tin situ and in real time) the flow-induced microstructure of immiscible polymer blends. Dilute mixtures of poly(dimethylsiloxane) in poly(isobutene) and vice versa have been used as model systems. It is demonstrated that different mechanisms for structural changes can be identified with this technique: droplet deformation, droplet retraction, fibril break-up and coalescence. Time-resolved in-situ SALS measurements support the structural interpretation of the dichroism data. Breaking up of stretched filaments by Rayleigh instabilities can be deduced from dichroism measurements. The resulting strings of droplets can be detected with SALS. The time scales for the structural changes can be determined by dichroism, which makes it possible to study in situ the effect of the various process and material parameters. As an illustration, the effect of shear rate on fibril break-up and on droplet coalescence after sudden changes in shear rate has been considered. Break-up times decreased only slightly with increasing shear rate; coalescence times scaled with the square of shear rate rather than with strain under the conditions of the present experiments. (C) 1998 Elsevier Science Ltd. All rights reserved.