The possibility of acetone shock loadings to phenol acclimated systems resulting in sequential substrate utilization and increased effluent phenol concentrations was evaluated. Phenol acclimated batch and continuous-flow systems, developed with seed from a municipal wastewater treatment plant, were shock loaded with acetone, bacto-peptone, and domestic primary effluent. Phenol and acetone utilization rates were then monitored using direct injection gas-liquid chromatography. The results of the investigation indicated that, under the described experimental conditions, qualitative shock loading of phenol acclimated/utilizing cultures had no significant effect on effluent phenol concentrations. Variations of system pH, however, were found to have extreme effects.