This publication continues previously published work from the authors which evaluates the storage potential of poultry fat for its use as an unmodified biofuel in heat and steam generating systems [ 1]. The previous work examined the physical properties of these fats while this study is concerned with chemical properties. In this study poultry fat (biofuel) and its 20%, 40%, 60% and 80% mixtures with #2 pump diesel fuel were stored for 1 year at bench scale (1L) under controlled laboratory conditions at 4, 38, 54.4 degrees C and at 22 degrees C. One hundred percent of poultry fat was studied under these same conditions with and without an antioxidant additive. Twenty and eighty percent of poultry fat mixtures were also stored at pilot scale (250 gallons) under outdoor, ambient conditions. Chemical properties relevant to the use of these mixtures as a biofuel for industrial boilers were studied and tracked. These properties include Energy Values (BTU), Ultimate Analysis (carbon, hydrogen, oxygen, ash, nitrogen, and sulfur), moisture, impurities, unsaponafiables (MIU), and Free Fatty Acid Content. Energy content of the biofuel samples dropped over the course of the study with untreated biofuel losing 18.9% of its BTU value over the course of a year. Ultimate analyses showed an overall increase in carbon composition and a decrease in oxygen content. Hydrogen levels increased in most treatments as did overall ash and sulfur percent composition. MIU values increased, this overall increase was attributable to variable increases in insolubles and unsaponifiables. Free fatty acid levels increased in all treatments. Most of the chemical changes observed in this study were reduced by the addition of antioxidant to 100% biofuel. (C) 2009 Elsevier Ltd. All rights reserved.