Several semi-continuous series of acetic acid fermentations were carried out in a novel pilot plant reactor (225 1) including a gas recycling system for the recovery of volatile compounds. The behaviour of substrate and product concentrations, acetification rates, stoichometric yields and oxygen consumption were analysed. Operation with this novel reactor proved some technical advantages in order to produce high quality vinegars: stoichometric yields of 100% and minimum oxygen supply. (C) 2003 Elsevier Ltd. All rights reserved.