The effect of shear flow on the miscibility behaviour of the model polymer blend polystyrene/poly(vinyl methyl ether) has been investigated in real time by two-dimensional light scattering and optical microscopy. Under certain shear and dynamic temperature conditions large ’waves’ have been observed which are aligned either in the direction parallel to the applied shear flow or just off it. Two-dimensional light scattering patterns from samples under the same conditions show sharp streaks aligned in the direction perpendicular to the flow. The evidence presented in this work indicates that these ’waves’ are unlikely to be elongated phase-separated domains; instead, they could be caused by fluid instabilities.