The production and emission of volatile organic compounds (VOCs) were studied using an online monitoring method in a well-operated sludge composting plant. Results indicated that VOC production within the pile was different from emission at the pile surface. The total mass of VOC production was 1.09 g C kg DM-1, which was 2.3 times as high as the total mass of emission. The maximum production and emission masses occurred in the mesophilic phase of composting and were 444 and 202 mg kg DM-1 d(-1), respectively. VOC production and emission rates also varied rapidly at different times. The relationship of VOC production rates and time in an on/off aeration cycle at different periods could be expressed as a quadratic equation, while the emission rate could be expressed as a linear equation. (C) 2012 Elsevier Ltd. All rights reserved.