Solutions prepared by suspending freeze-dried sludge in water were found to contain aggregated particles of about 1000-nm size and substantial concentrations of surfactants. The microparticles sorbed significant amounts of the organophosphorus (OP) pesticide diazinon, causing low recoveries and a decrease in observed partition coefficients with an increase in the suspended sludge concentration. The microparticles could be removed by prewashing the sludge, by filtration, or by passing the solution through a C-18 cartridge, which effectively acted as a filter. In contrast to diazinon, the recovery of the more polar OP propetamphos was excellent. Microparticles were not significant in muffled sludge obtained from the same source but were significant in three waste streams at a sewage treatment plant, including the final effluent. This could pose problems when predicting the extent of pesticide removal during sewage treatment.