The molecular weight, MW, distribution of soluble microbial products, SMP, was examined. Phenol, an inhibitory substrate, and glucose, a non-inhibitory substrate, were degraded using acclimated cultures of bacteria. Three distinct regions were found to exist, Region I: Original substrate present, Region II: Biodegradable SMP present, and Region III: Endogenous respiration. Phenol degradation resulted in more SMP than glucose, about 25 percent versus 3 percent as residual SMP at the end of Region I, and 3 percent versus 1 percent at the end of Region II, respectively. In Region III, the production of SMP due to endogenous decay, SMPE, was proportional to the rate of cell degradation. The rate coefficient for SMPE production for cells grown on phenol was higher than for glucose, 0.005 mg SMPE per mg cell carbon per day for phenol versus 0.002 mg per mg per day for glucose. Although differences existed in the magnitude of SMP produced, the MW distributions for phenol and glucose were similar in each region. While in Region I most of the SMP consisted of the lowest MW (<1 K daltons) compounds, 90 percent for phenol and 75 percent for glucose, at the end of Region II only 41 percent of the SMP for phenol and 56 percent for glucose were in the <1 K fraction. Finally, for endogenous decay products, 48 and 50 percent of the SMPE were in the highest MW fraction >100 K.