Biodegradation of nonylphenol polyethoxylates (NPEOs) by denitrifying activated sludge was investigated. The results showed that NPEOs were readily degraded in the denitrifying activated sludge process. Organic substance, initial concentration, and temperature had great influence on biodegradation of NPEOs in the denitrifying activated sludge process while the influence of biodegradation intermediates such as nonylphenol (NP) could be neglected. Biodegradation of NPEOs was severely inhibited in the presence of organic substances. Different organic substances had different inhibition ability on the biodegradation of NPEOs. The maximum biodegradation rate increased 2.51 microM d(-1) for each 10 microM increase in initial concentration of NPEOs. This linear relationship was maintained even at relatively high initial concentration. The decrease in temperature caused a sharp decrease in the removal efficiency of NPEOs. The temperature coefficient (Phi) for the biodegradation of NPEOs in the denitrifying activated sludge process was 0.011 degrees C(-1). NPEOs were biodegraded through a nonoxidative pathway, through which NPEOs were degraded via sequential removal of ethoxyl units (as acetaldehyde) to NP. Compared to anaerobic activated sludge treatment, denitrifying activated sludge treatment had much higher removal efficiency of NPEO contaminants. To our knowledge, it is the first report on the biodegradation of NPEOs in denitrifying activated sludge process.