AbstractSimultaneous wet milling and enzymatic hydrolysis of newsprint in a ball mill reactor (BMR) and in an attrition mill reactor (AMR) were studied. The effect of milling on the enzyme deactivation was also investigated. Variables studied included milling media size, material, and the presence or absence of an air–liquid interface in the reactor. At a reaction temperature of 45°C and a pH of 4.5 and in the presence of an air–liquid interface, the amount of reducing sugars produced by hydrolysis using a BMR was typically 5 to 15%less than the amount produced using an AMR. Without an air–liquid interface there was no significant difference. The effect of an air–;liquid interface on enzyme deactivation without any substrate was also studied. Enzyme activity after six hours of milling ranged from 1 to 85% ofthe starting activity. In both the AMR and the BMR, the extent of enzyme deactivation was 20 to 50% greater with an air–liquid interface present than with the air–liquid interface absent.