The lightest-molecular-weight substances in this group are highly flammable and pose significant vapor cloud explosion hazards (examples are ethylene and propylene). All are combustible.

Unsaturated aliphatic hydrocarbons are generally much more reactive than alkanes, which are saturated aliphatic hydrocarbons. Strong oxidizers may react vigorously with them. Reducing agents can react exothermically to release gaseous hydrogen gas. In the presence of various catalysts (such as acids) or initiators, compounds in this class can undergo very exothermic addition polymerization reactions. Many of these compounds undergo autoxidation upon exposure to the air to form explosive peroxides (this process generally occurs slowly). These peroxide and polyperoxide substances are usually extremely unstable and prone to detonation. The peroxidation of butadiene has been involved in several serious industrial explosion accidents.

Unsaturated aliphatic hydrocarbons have low toxicity, but act as asphyxiants.

Unsaturated aliphatic hydrocarbons are characterized by straight or branched carbon chains containing at least one double or triple bond between the carbon atoms. They are also known as alkenes or olefins (for compounds containing a C-C double bond) and alkynes (for compounds containing a C-C triple bond). Their physical state at room conditions changes with increasing molecular weight from gaseous to waxy solid. They are used in making rubber and plastics and in organic synthesis.

Butene, pentene, acetylene, amylene, cycloheptatriene, cycloheptane, cyclohexane, cyclooctadiene, isobutylene, dipentene, ethylene, hexene, isohexene, isoprene, methyl pentadiene.