Biofuels are part of the current energy mix and their contribution will increase in the years to come. However, policymakers and NGO have concerns regarding their sustainability. An adequate sustainability indicator is the energy return on investment (EROI), which evaluates the ratio of fossil energy consumption during their production. This paper applies the Exergy Cost Theory to different biodiesel production pathways, from crop cultivation to the production plant, in order to check if this principle is achieved using exergy instead of energy. For this purpose, it introduces a new sustainability indicator, called ExROI, defined as the ratio between the exergy of a particular resource and the amount of exergy required to obtain it. In other words, using exergy terms it tries to answer to the question: How much biodiesel would be needed to produce 1 kg of biodiesel? The paper demonstrates that the usual biodiesel production pathways are sustainable in this sense, although measures must be taken to decrease the rate of non-renewable energy consumption in the process, in order to make them economically competitive too. (C) 2012 Elsevier Ltd. All rights reserved.