Textile (eg cotton) finishing industry wastewater is characterised by high concentrations of surfactants (up to 2 g dm(-3)) and of readily biodegradable biopolymers (COD 5-15 g dm(-3)). The anionic surfactant decyl sulfate (DS) was chosen as model surfactant and soluble starch (size) as model compound for the readily biodegradable fraction of the wastewater. Twenty-two batch experiments with increasing DS/biomass ratio (and starch/biomass ratio) were started simultaneously. Biomass concentrations ranged from 50 to 15000 mg dm(-3). Minor inhibition effects were found for the surfactant degradation itself at all DS/biomass ratios (maximum biodegradation rate 7.7 mg(DS) g(biomass)(-1) h(-1)). The starch hydrolysis started without a lag-phase at DS/biomass ratios of up to 0.15 g(DS) g(biomass)(-1). The lag-phase was prolonged to about 100 h at a very high DS/biomass ratio (3 g(DS) g(biomass)(-1)), The relative importance of the accumulated intermediates was dependent on the DS/biomass ratio. Above 0.3 g(DS) g(biomass)(-1) 10% of the substrate organic carbon accumulated as ethanol, but no ethanol accumulation was observed at low DS/biomass ratios. Moderate DS/biomass ratios caused a considerable delay of the methanogenesis; high DS/biomass ratios prevented the methanogenesis almost completely.