Asphalts are highly complex and not well-characterized materials containing saturated and unsaturated aliphatic and aromatic compounds with up to 150 carbon atoms. Their composition varies depending on the source of crude oil. Many of the compounds contain oxygen, nitrogen, sulfur, and other heteroatoms. Asphalt typically contains about 80% by weight of carbon; around 10% hydrogen; up to 6% sulfur; small amounts of oxygen and nitrogen; and trace amounts of metals such as iron, nickel, and vanadium. The molecular weights of the constituent compounds range from several hundred to many thousands.

The compounds are classified as asphaltenes or maltenes according to their solubility in hexane or heptane. Asphaltenes are high molecular weight species that are insoluble in these solvents, whereas maltenes have lower molecular weights and are soluble. Asphalts normally contain between 5 and 25% by weight of asphaltenes and may be regarded as colloids of asphaltene micelles dispersed in maltenes.

Many of the compounds in asphalt are polar since they contain alcohol, carboxyl, phenolic, amine, thiol, and other functional groups. As a result of this polarity, the molecules self-assemble to form multimolecular clusters with molecular weights up to 100,000. The adhesion of asphalt to aggregate is also thought to depend on the polar attraction between molecules in asphalt and the polar surfaces of aggregates.