The chemistry of ANFO detonation is the reaction of ammonium nitrate with a long-chain alkane (CnH2n+2) to form nitrogen, carbon dioxide, and water. In an ideal stoichiometrically balanced reaction, ANFO is composed of about 94.3% AN and 5.7% FO by weight. In practice, a slight excess of fuel oil is added, as underdosing results in reduced performance while overdosing merely results in more post-blast fumes. When detonation conditions are optimal, the aforementioned gases are the only products. In practical use, such conditions are impossible to attain, and blasts produce moderate amounts of toxic gases such as carbon monoxide and nitrogen oxides (NOx).
The fuel component of ANFO is typically diesel, but kerosene, coal dust, racing fuel, or even molasses have been used instead. Finely powdered aluminium in the mixture will sensitise it to detonate more readily.
ANFO is classified as a blasting agent, meaning that it decomposes through detonation rather than deflagration at a velocity higher than the speed of sound in the material but cannot be detonated with a No. 8 blasting cap without a sensitizer. ANFO has a moderate velocity compared to other industrial explosives, measuring 3,200 m/s in 130 mm (5 in) diameter, unconfined, at ambient temperature.
ANFO is a tertiary explosive, meaning that it cannot be set off by the small quantity of primary explosive in a typical blasting cap. A larger quantity of secondary explosive, known as a primer or a booster, must be used. One or two sticks of dynamite were historically used; current practice is to use Tovex or cast boosters of pentolite (TNT/PETN or similar compositions).