The structure of dispersions of sterically stabilised poly(methyl meth
acrylate) (PMMA) particles under flow conditions are presented. Data h
ave been obtained with a light scattering apparatus in which the sampl
e can be sheared. The colloidal dispersions were prepared from particl
es with a significant density difference to the suspending medium whic
h gives rise to sedimentation. The scattering patterns indicate fluid-
like structures coexisting with ordered arrays of particles. The order
ed phase, which has a higher number density than the fluid, is found t
o undergo significant structural changes under steady shear. Diffracti
on from ordered and disordered layers of particles in finite-sized sta
cks is discussed. Quantitative interpretation of the scattering patter
ns in terms of particle arrangements are presented. The sedimentation
gives rise to density and velocity gradient variations across the samp
le with slip regions. Such important effects would go undetected in rh
eological measurements, leading to misinterpretation of the rheologica
l behaviour.