Universally valid expressions for the momentum and mass diffusion coefficients in stratified shear flows do not exist. Similarly, the effect of molecular diffusion in the mixing effciency of a stably stratified has not been investigated previously. In this paper the mixing efficiency in stably stratified flow is considered. Numerical investigations of unsheared stratified flows provide qualitative insight into the turbulence characteristics of stably stratified turbulent flows. Different types of stratified shear flows give qualitatively similar results for the mixing efficiency in the gradient Richardson number, Ri, range of 0.01–1.0. Mixing efficciency is proportional to Ri and its magnitude appears to depend on stratification in the fluid column. Suspended solids appear to effect the turbulence exchange processes in a different manner to those of salt and heat. The ratio of the diffusivity of solute (or mass) to the diffusivity of momentum is less than 1 and can be taken as 0.25 for sediment-laden turbulent shear flows. This ratio is about a factor of 2–3 larger when heat- and salt-induced stratification exist, the difference possibly due to the settling nature of suspended solids.