A commercial form of hydrocyanic acid (or a hydrogen cyanide) that vapourizes when exposed to the air, manufactured by the firms Degesch and Tesch/Stabenow, under patent from I.G. Farben, brought to Auschwitz in the summer of 1941 as a disinfectant and insecticide (for delousing purposes) but subsequently applied as a means of mass execution more efficient than the redirected-exhaust container trucks which were then in use. By the end of the war Degesch was supplying two tons of Zyklon-B (Cyclone-B) a month and Tesch another three-quarters of a ton. By then the gas had been used to exterminate an estimated six million prisoners, primarily Jews.

Prisoners were led into gas chambers which had been disguised as public showers. When they were all in the room, the chamber would be made airtight and Zyklon-B would be released into the air, vapourizing and giving off cyanide's characteristic bitter almond smell.

The vapours combined with red blood cells, resulting in oxygen starvation. The particular brand of Zyklon-B used by the Nazis in the death camps contained blue colouring and blue stains can still be observed today in untampered gas chambers.

Death came almost instantly to those located adjacent to the air vents through which the Zyklon-B was introduced; the entire chamber would have been dead within fifteen minutes. After a half-hour, forced ventilation was turned on and the Sonderkommandos would enter the room, removing gold teeth and cutting the hair from the women, after which the corpses were transported by elevator to the ovens where they were cremated.

Though the German companies that made and sold the gas to the Nazis claimed after the war that they were unaware of their product's potential as a means of human execution the gas had been used in execution gas chambers since 1920 in the USA, when its use began in an Arizona prison.