A method for determining the reactant concentration and biocatalyst activity during a biocatalysed reaction is described. The analyser functions by monitoring the temperature change of a sample of reaction mixture which is held under adiabatic conditions. Details are given of the successful application of the analyser to the monitoring and control of a fed-batch bioconversion by Rhodococcus ruber NCIMB 40757 of acrylonitrile to 42% (w/w) (4.37 M) ammonium acrylate and the wider application of the analyser to other bioconversions is considered.