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High operational stability and productivity of co‐immobilised systems are important aspects for their successful application in industrial processes. A dynamic model is required to describe artificially co‐immobilised systems because the time needed to reach steady state normally exceeds the operational life span of these systems. Time dependent intraparticle concentration profiles and macroscopic conversion were modelled to study the operational stability and productivity of these systems theoretically. The model was used to describe experimental results of ethanol production from maltose by a co‐immobilised system of amyloglucosidase and Zymomonas mobilis. Furthermore, the influence of the immobilisation procedure with glutaraldehyde and polyethyleneimine could also be studied with and incorporated in the model. From the model it could be derived that co‐immobilised systems performing a consecutive reaction evolve towards a steady state, characterised by a constant concentration of the intermediate in the particle if product inhibition is neglected. Such a situation develops independently of the biomass concentration and the radial position, and has important consequences for co‐immobilised systems. When the concentration of the intermediate in the bulk liquid is lower than this constant value in the biocatalyst particle, two regions may be distinguished in the particle: an inactive peripheral region without biomass and an active core with a biomass concentration depending on the substrate and immobilised enzyme concentration. Unlike immobilised single cell systems, it is possible to obtain a real steady state and therefore a stable situation for co‐immobilised systems. However, a high operational life time could only be achieved at the expense of the productivity of the biocatalyst particle. A stability criterion is derived which agrees very well with the simulation results. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 63: 694–704, 1999.

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