Ethyl formate and carbonyl sulfide are potential stored product fumigants. They are present naturally in the atmosphere, plants and soil. Natural levels of ethyl formate and carbonyl sulfide in newly harvested grain and stored wheat, barley, oats and canola are varied with the commodity, temperature, moisture content and the period of storage. The values ranged from 0.5~2.0 mg/kg for ethyl formate and 0.02~1.0 mg/kg for carbonyl sulfide. Ethyl formate and carbonyl sulfide were naturally present in grains at harvest, increased during the first 4~5 months of storage, and then began to decline, particularly at grain temperatures greater than 20 ℃ and moisture contents of grain and canola greater than 9.5% and 5%. This information on natural levels is relevant to the establishment of Maximum Residue Limits (MRL) and to market acceptance of ethyl formate and carbonyl sulfide as fumigants for stored products.