Furan is classified as “reasonably anticipated to be a human pathogen” by the U. S. Department of Health and Human Services. A recent survey by the Food and Drug Administration revealed furan is present at levels as high as 100 ppb, in a wide variety of foods that undergo heat treatment. Our recent studies suggested that ionizing radiation, a nonthermal processing technology, can induce low ppb levels of furan in fruit juices. The mechanism of furan formation due to the two processing technologies was, however, unclear. Therefore, we conducted a study to investigate the effect of thermal treatment and ionizing irradiation on formation of furan in common juice components such as sugars, ascorbic acid and organic acids. Solutions of sugars, ascorbic acid and organic acids were treated with different doses of gamma rays or heating conditions. Furan in the solutions was measured using GC-MS. Furan was found from solutions of ascorbic acid, fructose, sucrose, or glucose after thermal treatments and irradiation. No furan was produced from malic acid or citric acid. The pH and concentrations of sugar and ascorbic acid solutions had profound influences on furan formation. The rate of irradiation- or heat- induced furan formation generally increased with decreasing pH. The results suggest that food compositions can play an important role in the formation of furan due to ionizing irradiation and heating treatment. The information will be useful for food industry to develop and optimize food formulations in order to minimize formation of this compound.