A simple, fast method for estimating the mean rate of biological oxidation in a typical semi-continuous wine vinegar production cycle was developed. The method involves on-line monitoring changes in ethanol concentration during the cycle and allows the acetification rate in each cycle step to be estimated. This datum is of a high interest with a view to optimizing the process by establishing the influence of the operational variables on productivity. (c) 2006 Elsevier Ltd. All rights reserved.