This article reports a novel nondisruptive technique for measuring the thicknesses of membrane-attached biofilms in situ, using a single tube extractive membrane bioreactor (STEMB). The biodegradation of a toxic volatile organic compound (VOC) (1,2-dichloroethane [DCE]) by Xanthobacter autotrophicus GJ10 has been used as a model system to develop the technique. The results give information on the biomass-silicone rubber attachment phenomena, and on the development over time of biofilms growing on the silicone membrane, without disrupting operation. Experimental results are presented showing the evolution over time of biofilm thickness, and also the density of biofilms for four experimental runs. The hydrodynamic conditions on the biomedium side of the membrane were found to influence the initial attachment phenomena and subsequent biofilm growth.