Biodiesel is a clean renewable fuel with many environmental advantages. One of the most important is its good biodegradability and ability to dissolve crude oil and its derivatives. In this paper, we used the CO2 evolution test to study the biodegradability of biodiesel and its mixtures with fossil diesel fuel and gasoline. The biodegradability of biodiesel was higher than 98% after 28 days, while for diesel fuel and gasoline it was 50% and 56%, respectively. In all the cases, biodegradability increased with the addition of biodiesel. To evaluate the synergic effect, the experimental results for the mixtures were compared with a linear combination of the biodegradability values for the pure compounds. The synergic effect was positive in all the cases, demonstrating that biodiesel enhances the biodegradability of both diesel fuel and gasoline by means of cometabolism. The density and viscosity of the mixtures were also evaluated to determine the possibility of collecting them from an affected area. Although both properties increased with the addition of biodiesel, in all the cases the density was lower than that of water and the viscosity was low enough to allow pumpability. (c) 2006 Elsevier Ltd. All rights reserved.