Biofilms play an important role as sorbents in the process of distribution of pollutants in surface water systems. The sorption properties of biofilms will influence the kind and the amt. of sorbed substances. The heterogeneity of biofilms provides different sorption sites which exhibit a different sorption preference and capacity. As dynamic systems, biofilms will respond physiol. to their environment. Thus, the sorption of one substance may lead to a change in the compn. of the EPS or other biofilm components and further alter the original sorption properties. In this paper, the influence of toluene on a biofilm was investigated. As a suitable method, FTIR-ATR spectroscopy was applied. The method is nondestructive and allows the observation of biofilm formation and behavior online and in situ. A biofilm was allowed to form in ATR flow-through cells. The test strain was genetically engineered and contained a bioluminescent reporter gene which was switched on when toluene was metabolized. Thus, the degree of toluene degrdn. activity could be obsd. with great sensitivity. The FTIR spectrometer contained three flow-through cells which could be operated in parallel: one was run with sterile medium only, one with medium and bacteria, and one with medium, bacteria and toluene. This arrangement allowed the discrimination of the biofilm response from other effects. The IR spectrum showed specific bands of proteins, polysaccharides, phosphoryl compds. and other groups of mols. A significant increase of EPS-polysaccharide formation was obsd. at a toluene level of 5 mg/L. At 15 mg/L, significantly more carboxyl groups were formed. Thus, the effect of the lipophilic org. pollutant toluene increased the amt. of neg. charged groups and, consequently, the sorption capacity for metal cations. This result indicates that biofilms respond in a complex manner to different sorbates and alter their environmental properties.