Fig. 4. Decision flow chart on which biphasic solvent system is suitable for a biocatalytic reaction. VII: This can happen if the substrate or product concentrations will accumulate in the aqueous phase to a point they will act as an inhibitor or will negatively influence the protein structure. If this is the case, a second phase is required to act as reservoir for the compounds and to decrease aqueous concentrations of the compounds. VIII: Also in the case of aqueous-neat reaction conditions, some cosolvents can be added to increase the substrate concentration in the aqueous phase, if beneficial for the reaction. IX: If the required biocatalytic rates are significantly higher than the mass transfer rates, the reaction is not working optimally and effort should be put in increasing these transfer rates. A convenient way to test if transfer rates are limiting, is to increase the biocatalyst concentration and check if the reaction rate is increasing accordingly.