The objective of this study is to determine the inert soluble COD of wastewaters from the fermentation industry. In this context a series of experiments were performed for various effluents from baker's yeast industry including raw process wastewater, anaerobic pre-treatment plant effluents, domestic and washing waters mixture. The inert COD ratio (S-1/S-0) for the raw effluents from baker's yeast industry was determined as 0.1. This ratio was in the range of 0.20 to 0.30 for the anaerobically pre-treated effluents. The S-1/S-0 ratios for the wastewater simulating the effluent of the existing full-scale aerobic treatment plant have varied from 0.18 to 0.48. Such a large variation has been originated from the operating conditions of the existing full-scale anaerobic treatment plants. The higher volumetric loading rates and shorter sludge retention times correspond the lower S-1/S-0 ratios for the full-scale anaerobic treatment systems in general. Copyright (C) 1996. Published by Elsevier Science Ltd.