It is well known that the rates of fast, multiphase reactions, that occur in the diffusion zone near the interface, may be enhanced by the use of an additional, emulsified phase, which shows a high affinity for the rate limiting solute. However, the effect of such an additional phase on the rate behaviour of systems where slow reaction occurs dominantly in the bulk, well mixed region has not been assessed so far. This effect can be of great significance, especially in situations, where the reaction kinetics depend upon the reactant concentration in a complex way (e.g. substrate inhibition) and may be of particular importance in biochemical systems. In this study, the conversion behaviour of a model steroid system, namely, the biphasic, enzymatic conversion of testosterone in a batch reactor, has been explored in the presence of an emulsified oil phase. The kinetic rate of reaction of this species displays a maximum with respect to its concentration. It has been shown that the presence of an emulsified liquid phase can improve the substrate conversions significantly and that the extent of such an improvement depends upon the solubilizing capacity of this phase as well as its hold-up. The use of a microheterogeneous media, in lieu of a plain reactive liquid, can therefore be used to drive the reactor towards optimal behaviour with a view to maximizing the substrate conversions.