Aqueous solutions containing high concentrations (circa 1000 mg/L) of benzene can be treated biologically through the intervention of organic 'sponges'. In reality these sponges are immiscible and biocompatible organic solvents that can be added in very low volumes, and act to draw benzene out of the aqueous phase, reducing levels appropriate for biological treatment. As the organisms consume benzene, the sponges release additional substrate to maintain an equilibrium relationship between the two phases, and this rate is determined by the metabolic activity of the cells. We have used 1-octadecene as the organic sponge, and Klebsiella sp. as the degradative organism to consume 1000 mg/L of benzene in 12 h. By draining the aqueous phase to 10% of its original volume (and letting it serve as an inoculum), additional benzene solutions, at 1000 mg/L, can be reintroduced to the system, and the action of the sponge used a second, and subsequent times, to control benzene levels and benzene delivery to the organisms.