Winery wastewaters are characterised by seasonality and variable volume and organic load. This fact together with their high content of biodegradable compounds often results in problems in the operation of biological systems as they may lead to poor sludge settleability, floc disintegration and increased presence of solids in the treated effluent. Biodegradation of winery wastewater from a Port Wine production industry was studied in aerobic batch assays, varying substrate and biomass concentrations. More than 90 % of COD was removed in all cases, in a short period when biomass concentration was higher than 3 g VSS L–1. Data was correlated to several kinetic models, and Haldane model best fitted the experimental data, particularly for lower biomass concentrations. Therefore, an initial high biomass concentration should be present in aerobic treatment of winery wastewater, in order to cope with the large fluctuations in their organic loads. These batch assays are valuable for winery wastewater treatment, as they may simulate typical start-ups after short and long shutdown periods often observed in the winery industry.