A biodiesel storage stability study was conducted on Ultra Low Sulfur Diesel Fuel (ULSDF) and three biodiesel fuels (B100), including a Tallow-based Methyl Ester (TME), a Canola-based Methyl Ester (CME), and, a Yellow Grease Methyl Ester (YGME), and fuel blends (B5 and B20). The stability study was conducted over ten months (Aug 07-Jun 08) and consisted of storing fuel samples in quarter filled plastic and steel 20 L cans, in an unheated shed. Fuel cans were vented twice a week to ensure exposure of the fuel to air. Changes in Acid Number, kinematic viscosity and free water and sediments levels were monitored over the storage period. Temperature and relative humidity ranged from -25 degrees C to 35 degrees C and 25% to 100%, respectively. Acid Number and the viscosity did not increase beyond the uncertainty of the method used. Reference samples kept at 40 degrees C were used for comparison. Free water and sediment levels were barely above the detection limit of 0.003 mL/100 mL of fuel for CME and YGME and their blends. TME and its B20 blend displayed free water and sediment levels up to 0.05 mL/100 mL of fuel in February, after six months of storage. These values decreased considerably after the warm Summer months back to below 0.01 mL/100 mL. All free water and sediment levels were measured after fuel samples came to equilibrium with room temperature. The TME B5 free water and sediment levels remained low throughout the storage period. Proton Nuclear Magnetic Resonance (NMR) spectroscopy performed on particulates from the sediments revealed a similar composition but a slightly lower concentration for protons in alkenyl groups. The presence of these sediments was attributed to more saturated molecules coming out of solution in colder weather and very slowly going back to solution at room temperature. (C) 2010 Elsevier Ltd. All rights reserved.