The use of ozonized vegetable oils as pour point depressant for neat biodiesel was evaluated. Ozonized vegetable oils (1-1.5% by weight) were effective in reducing the pour point of biodiesel prepared from sunflower oil, soybean oil and rapeseed oil to -24, -12 and -30 degrees C, respectively. Cloud point however remained unaffected. In the case of palm oil biodiesel, significant reduction was observed in cloud point but not in pour point. Statistical analyses showed that neat biodiesel and biodiesel treated with ozonized vegetable oils showed no significant difference in other properties including density and viscosity. Although ozonized vegetable oils increase the flash point of biodiesel, the values are still within the limits set by the standards in the US and Europe. Lowest reduction in pour point was observed in cases where the biodiesel and the ozonized samples were prepared from the same vegetable oil. Hence, a correlation may exist between the nature of the biodiesel and ozonized oil. Microscopic analysis at low temperature revealed that ozonized vegetable oil impede agglomeration of biodiesel into network of solidified material giving crystals with sizes around 10 mu m. (c) 2005 Elsevier Ltd. All rights reserved.