Sorption, alkaline hydrolysis, photolysis and microbial transformation of diethyl phthalate (DEP) were studied in a large laboratory aquatic ecosystem composed of a series of reactor tanks. Microbial transformation accounted for 95 to 99% of the rapid loss of DEP from water. First-order rate coefficients for the loss of DEP, except in reactor tanks depleted of inorganic nutrients, were statistically constant among diverse chemical and microbial environments, suggesting a constant transformation rate per unit of colonized surface area. The diversities of reactor tank environments were substantiated by large ranges in the dissolved organic carbon, nitrogen, phosphorus, chlorophyll, ATP and biomass measurements. Microbial transformation of DEP resulted specifically from bacterial activity, most of which was associated with aquatic microbial growth (aufwuchs) attached to submerged surfaces or suspended in streamers or mats.