As the production of biofuels continues to expand worldwide, criticism about many issues, including the energy output versus input and the competition with food, has been raised andthe sustainability of biofuels in recent years has been constantly debated. However, the current biofuel systems may be optimized to increase the energy efficiency and environmentalperformance. By using concepts from industrial symbiosis, the material and energy exchangesmay be optimized to result in these performance improvements. This paper offers aquantification of the environmental performance of industrial symbiosis in the biofuelindustry through integration of biogas and ethanol processes using a life cycle approach.Results show that although increasing integration between the biogas and ethanol plants isassumed to produce environmental benefits, not all impact categories have achieved this andthe results depend upon the allocation methods chosen. Thus the increasing integration alsobrings about increased complexity for the system.