Legislative history of water pollution control has not included detailed scientific definitions of what is meant by the rather inclusive term “oil.” Because of the publicity surrounding spills of crude or petroleum-derived oils, little attention has been focused on non-petroleum oils. Approximately 5% of the oil spills reported to the Environmental Protection Agency are nonpetroleum oils. Their physical and chemical properties and adverse environmental effects are strikingly similar to the behavior of petroleum oil in the aquatic environment. This paper presents a comparative analysis of the properties and effects of petroleum versus nonpetroleum oils. Their similarities prove that these oils should be treated as one entity regardless of their origin. Finally, additional guidelines are presented to provide a more broadly applicable distinction between oil and other hazardous materials for enforcement purposes.