The enzymes involved in the malting and mashing stages of malt whisky production act under changing conditions. Their actual activity is dependent upon their inherent properties and current process conditions and thus may differ from those observed under optimum conditions. A range of malted barley enzymes has been measured under optimal conditions and process conditions at different stages of malt whisky production. In laboratory mashing conditions, alpha- and beta-glucosidases, alpha- and beta-galactosidases, alpha- and beta-mannosidases N-acetyl-glucosaminidase and arabinosidase were between 24-82% lower in activity in process conditions. Alpha-amylase was the same and beta-xylosidase was little affected. In the mash run, the residual reduction in activity was bent een 20.3-91.2% for most enzymes while beta-glucosidase and alpha-mannosidase were largely unaffected. After a 34 h fermentation, the percentage reduction compared to the optimum conditions was between 10.4-95.4%.