A stable water-in-oil-in-water (W-1/O/W-2) double emulsion containing in its inner aqueous phase a microbial consortium (60 mg(protein) L-double emulsion(-1)) known to degrade methyl tert-butyl ether (MTBE) was prepared. The primary emulsion (W-1/O) had a volume disperse phase (phi(1)) of 0.3 and was stabilized with 8% Span 80. The double emulsion had a volume disperse phase (phi(2)) of 0.3 and was stabilized with 0.2% polyacrylic acid (PAA). A maximum degradation rate of 12.4 mg(MTBE)g(protein)(-1)h(-1) was achieved in a microcosm that contained an initial concentration of 146 mg(MTBE)L(double)(-1) emulsion. This degradation rate was significantly lower (P < 0.05) than that exhibited by the free cells (48.7mg(MTBE) g(protein)(-1) h(-1)), probably because the encapsulated cells metabolized an ingredient of the double emulsion as carbon source besides MTBE. Nevertheless, our results show that this biodegradation technique has great applicability potential. (C) 2008 Elsevier Ltd. All rights reserved.