Industrial fermentations carried out in a 500-1 bioreactor were monitored on-line by a prototype of a split-flow modified thermal biosensor. Acetaldehyde and glycerol in the extracellular broth were monitored over the first 48 h of fed-batch fermentations. The aim was to determine the usefulness of these secondary metabolites for on-line monitoring and control. When fermentation of the 13-16 g/l batch sugar was monitored, using immobilised aldehyde dehydrogenase, the acetaldehyde reached a peak value of 0.3 g/l. With immobilised alcohol oxidase a much larger peak of 3.5 g/l ethanol was seen immediately after the acetaldehyde peak. When glycerokinase was used a delayed peak of 1 g/l glycerol was monitored. Of the three metabolites monitored, the ethanol proved the most valuable indicator of suitable timing for the start of the feeding phase and later for controlling and preventing overfeed using the on-line biosensor system.