Under appropriate agitation conditions a suspension of particles in a liquid will form two layers with the particles entirely in the lower layer, yet fully suspended. This type of suspension can be utilized to protect an immobilized biocatalyst from deactivation by a substrate. If the substrate is added to the upper layer it mixes within that layer and the diluted substrate transfers to the lower layer which contains the biocatalyst. By simulating the suspension of an immobilized biocatalyst with that of an ion exchange resin it has been possible to investigate the characteristics of a two layer suspension and determine the rate of transfer of liquid between the two layers. This data has been used to predict the productivity of a bioreactor operating in the two layer mode