Alkenes fall between alkanes and aromatics in structure and properties. They are not found in crude oils, but in some refined products, such as gasoline, and in cracking products. Alkenes are probably more toxic than alkanes, but less so than aromatics.

Aromatic hydrocarbons are characterized by the possession of six member rings of carbon atoms which have three double carbon-carbon bonds. These are the "benzene-rings" familiar to basic chemistry students. Compared to alkanes, aromatics are not common in nature. Low boiling aromatics are thought to be the most immediately toxic compounds found in oil. Found in virtually all oils, low boiling aromatics are quite soluble in water and can kill not only at full strength, but in dilutions. Higher boiling aromatics, especially multi-ring compounds, are suspected as long-term poisons and some are known carcinogens ().

The biological impact of an oil spill depends in part on the type and amount of oil spilled, and the amount of change the oil has undergone while in or on the sea. For example, light fuel oils, having more aromatics than heavy fuel oil, will have more toxic effects. On the other hand, heavy fuel oil might do more mechanical damage to intertidal life by smothering or physically removing organisms. Indeed, these differences have been used to explain the disparity in observed effects of oil spills. For example, Straughan () has suggested that the devastating effects of the Tampico Maru and West Falmouth spills were due to the fact that light fuel oils with high concentrations of volatile aromatics were spilled. She contends that crude oil spills like that at Santa Barbara are not as devastating because of the lower toxicity of the oil reaching shore.