Soluble microbial products (SMP) generated by activated sludge cultures receiving a mixed feed of phenol and glucose were characterized with respect to molecular weight (MW) distribution, octanol-water partition coefficient (K-ow), and Microtox (R) toxicity. Short-term batch reactor tests using C-14-labeled substrates were performed to collect SMP derived from each substrate, while longterm tests were performed with SMP accumulated over multiple feed cycles using fed-batch reactors receiving non-labeled substrates. Yield of SMP in the batch tests, 10%-20% for phenol and 2%-5% for glucose, differed for each substrate and was independent of initial concentration. The MW distribution (MWD) of SMP was independent of feed composition, and was bimodal in the < 1 kDa and 10-100 kDa MW ranges for phenol-derived SMP and predominantly < 1 kDa for glucose-derived SMP. In the non-labeled tests, the fraction of SMP of MW > 100 kDa increased with the proportion of glucose in the feed. The K-ow of phenol-derived SMP was higher compared to glucose-derived SMP, indicating that the phenol-derived SMP were more hydrophobic. This was particularly true at an acidic pH, where the K-ow was 4.2 +/- 1.0 for phenolderived SMP versus 0.13 +/- 0.13 for glucose-derived SMP. Toxicity testing indicated that phenol-derived SMP, exerting a mean Microtox (R) inhibition of 1%, were less toxic than phenol itself, and showed little correlation between toxicity and concentration. However, glucose-derived SMP were generally more toxic than glucose itself (a non-toxic substrate), and the toxicity increased linearly with the concentration of SMP.(c) 2006Wiley Periodicals, Inc.