A method for determining diffusion coefficients during sheat by dynamic light scattering is presented. The direct determination of diffusion coefficient during flow is hampered by the domination of the convection terms over the diffusion terms except in a very narrow azimuthal scattering angle range that is impossible to reach experimentally. The method described here offers the advantage to this constraint. The technique is based on a double measurement of homodyne spectrum of light scattered from small scatterers immersed in a medium subjected to a simple shear flow. This can be applied to polymer solutions. Verification is provided using suspensions of polystyrene spheres in Newtonian media and a transparent rheometer. (C) 1998 Elsevier Science Ltd. All rights reserved.