These oils are highly fluid, often clear, spread rapidly on solid or water surfaces, have a strong odor, a high evaporation rate, and are usually flammable. They penetrate porous surfaces such as dirt and sand, and may be persistent in such material. They do not adhere to surfaces. Flushing with water generally removes them. In addition to being flammable, these oils may be highly toxic to humans, fish, and other biota.

These oils have a waxy or oily feel. They are less toxic and adhere more firmly to surfaces than light crudes, although they can be removed from surfaces by vigorous flushing. As temperatures rise, their tendency to penetrate porous substrates increases and they can be persistent. Evaporation of volatiles may lead to a heavy, sticky oils or to non-fluid oil residues and tar balls. Medium to heavy paraffin-based oils fall into this class.

These oils are characteristically viscous, sticky or tarry, and brown or black. Flushing with water will not readily remove this material from surfaces, but the oil does not readily penetrate porous surfaces. The density of heavy crude oils may be near that of water and they often sink. Weathering or evaporation of volatiles may produce solid or tarry non-fluid oils and tar balls. Toxicity is low, but wildlife can be smothered or drowned when contaminated.

The best known light distillates are gasoline, naphtha and toluene. Since gasoline is in such widespread use, its characteristics should be noted. Gasoline is a lightweight material that flows easily, spreads quickly, and may evaporate completely in a few hours under temperate conditions. It poses a risk of fire and explosion because of its high volatility and flammability, and is more toxic than crude oil. Gasoline is amenable to biodegradation, but the use of dispersants is not appropriate unless the vapors pose a significant human health or safety hazard.

Middle distillates include kerosenes, jet fuel, light heating oils and diesel fuels. In widespread use is kerosene, a lightweight material that flows easily, spreads rapidly, and evaporates quickly. Kerosene is easily dispersed, but is also relatively persistent in the environment and very toxic.

The largest group of oil products is made up of the heavy distillates. These include heavy fuels, lubricating oils, transformer oils and hydraulic oils. No. 4 Fuel Oil is a medium weight material that flows easily and is easily dispersed if treated promptly. This fuel oil has a low volatility and moderate flash point, and is fairly persistent in the environment.

No. 5 Fuel Oil (Bunker B) is a medium to heavy weight material with a low volatility and moderate flash point. Preheating may be necessary to make it mobile in cold climates. This fuel oil is difficult to disperse. Trials in the Gulf of Mexico indicate that modern dispersant can disperse bunker.

No. 6 Fuel Oil (Bunker C) is a heavy weight material that is difficult to pump and requires preheating for use. This fuel oil may be heavier than water, is not likely to dissolve, is difficult or impossible to disperse, and is likely to form tar balls, lumps, and emulsions. It has a low volatility and moderate flash point.

Asphalt is a semi-solid sticky residue formed by the evaporation of the volatile oils that hold the residues in solution. It is the almost solid material that is left when oil is extensively weathered. It may be fluidized by heating. However, the cutback solvents that are use to keep it fluid during transport are flammable and toxic. If spilled into water, the cutback solvents will tend to float on, and dissolve into, the water while the heavier portion of the asphalt will tend to sink or in some cases remain suspended under the surface of the water.