A model has been developed to describe the interaction between two enzymes in a stirred vessel. Using the model, cofactor (NADPH) recycle has been investigated by simultaneous solution of the rate equations, solved with the aid of a numerical solution. The overall general aim of this work is to increase the information available to the designer on similar biocatalytic processes so that comparisons between process options can be quantified. This model allows the optimisation of the proportion of two enzymes and total cofactor concentration needed for a satisfactory reaction rate to be achieved. A secondary use of these data is in the prediction of cofactor stability which is a strong function of pH. The model also indicates the potential cofactor turnover number achievable, an important economic consideration. The accuracy of the model has been assessed and is in good agreement with experimental data. (C) 2000 Elsevier Science Ltd. All rights reserved.