Concern with environmental issues such as global climate change has stimulated research into the development of more environmentally friendly technologies and energy sources. One critical area of our economy is liquid fuels. Fast pyrolysis of lignocellulosic biomass for liquids production is of particular concern, as it is one of the most interesting ways to produce renewable liquid fuel for transport and heat and power production. The aerobic biodegradability of various pyrolysis oils from different origins and of a EN 590 diesel sample was examined using the Modified Sturm (OECD 301B). The results demonstrate that all fast pyrolysis oils assessed are biodegradable with similar shaped curves with 41-50% biodegradation after 28 days, whereas the diesel sample reached only 24% biodegradation. Since pyrolysis oils achieved biodegradability over 20% these are classified as inherently biodegradable. Modelling of biodegradation processes was successfully performed with a first-order chemical reaction. The biodegradability results obtained for biomass pyrolysis oils are compared to those of conventional and alternative fuels. (c) 2007 Elsevier Ltd. All rights reserved.