Laboratory experiments measured the effects of five alkylphenol polyethoxylate nonionic surfactants on the microbial degradation of glucose and pentachlorophenol (PCP) by a pure culture of Sphingomonas chlorophenolicum RA2 (RA2) that was unable to biodegrade the surfactants. The surfactants with mid-range hydrophile-lipophile balance (HLB) values of 13.5-15 were the most biocompatible with substrate degradation. Monomers of the surfactant with the lowest HLB value of 12.3 inhibited RA2 growth on both glucose and PCP. The surfactant with the highest HLB of 17.9 was only inhibitory to glucose biodegradation at 3000 mg/L, a concentration well above its CMC. The surfactants were more inhibitory of RA2 biodegradation of PCP compared to glucose, which is likely due to interactions with membrane-associated PCP-degrading enzymes rather than bioavailability limitations. These results may prove helpful in selecting surfactants for use enhancing surfactant-amended remedial applications involving biodegradation or oil dispersion. (C) 2012 Elsevier B.V. All rights reserved.