A novel submerged silicone membrane extraction technique for removing the dissolved gases, CO2 and H-2, was explored as a method for enhancing the stability of anaerobic digesters under shock loads. The effectiveness of this strategy was demonstrated during a five-fold step increase in the feed concentration from 4 kg m(-3) to 20 kg m(-3) Chemical Oxygen Demand (COD) to two anaerobic chemostats, a control and a membrane reactor, operating under steady state. Data revealed that membrane extraction of CO2 enabled neutral pH values to be maintained during the shock, in contrast to the control; and this appeared to enhance acetate degradation. In addition, while liquid phase hydrogen removal was only partially successful due to a biofilm building up on the membrane, it also appeared to enhance the rate of volatile fatty acid degradation, and hence improved reactor stability. Flee sizes were monitored during the course of the shock load experiment and found to decrease from 46 mu m to 30 mu m over 35 days in the control, but from 37 mu m to < 5 mu m in the membrane reactor. This decrease in hoc size was postulated to be due to the growth of a relatively pure methanogenic biofilm on the membrane.