According to sources, the key weapon to which the Byzantine navy owed its supremacy was Greek fire, also known as "sea fire ", "Medean fire" or "manufactured fire", which was the most highly refined version of hitherto known incendiary materials used for military purposes. People had known how to launch combustible liquids since ancient times, in the form of flaming arrows or flammable materials in containers. But what made Greek fire stand out was the fact that it did not extinguish on coming into contact with water.

According to the historian Theophanes, the inventor of Greek fire was the Greek-Syrian architect Callinikos , who equipped the ships that successfully defended Constantinople against the Arabs in 717-718. However, his contribution may have lain in improving the way the new weapon was propelled. The composition of Greek fire remains a mystery to this day, since both the ingredients and the preparation method were a state secret. Constantine Porphyrogenitus even threatened to excommunicate anyone betraying the secret formula, which was supposedly revealed to Constantine the Great by an angel sent from God. There is a kernel of history in this story, however, since the ingredients used for the manufacture of Greek fire were known since Antiquity. It was probably a mixture of naphtha , i.e. a flammable mineral oil like crude oil, and sulphur; quicklime, resin and other combustible substances would have aided ignition. Natural sources of naphtha were to be found in the area between the Caspian and the Black sea, as well as in Arabia.

Alongside the Byzantines, the Arabs had discovered and made successful use of incendiary mixtures both on land and at sea. These consisted mainly of naphtha and liquid tar, which could only be put out with sand, not water. Particularly during the first millennium, the two peoples were in constant contact - the rivalries between them, involving espionage, frequent battles and prisoners and weapons taken on both sides render the existence of truly secret weapons such as Greek fire highly unlikely.

Both Arab and Byzantine sources refer to similar ways of using Greek fire for military purposes. The liquid was kept in oblong clay or metal vessels called siphons. These were doused in the same mixture and set alight immediately before being launched, or wrapped in fabric soaked in the mixture, which was likewise set on fire. In some cases the siphons had fire torches that would have worked like modern day bomb fuses. The siphons were launched from special throwing machines located in the prows of ships. These machines were probably ballistae, i.e. large wooden structures that had a mechanism similar to a bow that fired stones or arrows. The ballistae were of various sizes, depending on the type or size of the ship using them; in at least one case they were covered with metal casts of wild beasts that spewed fire and smoke from their mouths. There were also “cheirosifones” (hand siphons) - probably small clay or metal pots filled with Greek fire, which would be thrown at the enemy like modern grenades.

The main success of Greek fire during naval battles was that it created confusion and panic among enemy ships, which were routed by fire and its deleterious effect on the morale of the crews in wooden ships. Greek fire was employed by the Byzantines up until the 13th century. The last mention of its use is made by Frantzis, historian of the Fall of Constantinople: liquid fire drove back the Ottomans who were digging tunnels under the city walls, and on 20^th April 1453 led to the loss of an Ottoman flotilla in the Bosporus, when used by Francesco Lecanella (Phlatanellas), governor of a ship chartered by Constantine XI Palaeologus.