Zyklon B also spelled Cyclon B or Cyclone B) was the trade name of a cyanide-based pesticide invented in the early 1920s, and manufactured by German chemical conglomerate IG Farben. Zyklon B consisted of hydrogen cyanide (prussic acid), a stabilizer, a warning odorant (ethyl bromoacetate), and one of several adsorbents. Zyklon A was a previously produced liquid pesticide, which released hydrogen cyanide in a chemical reaction with water. After the invention of Zyklon B, Zyklon A production ceased.

On September 3, 1941, about 600 Soviet prisoners of war and 250 sick Polish POWs were gassed to death with Zyklon B in Auschwitz Camp I.

This massacre came shortly after the first experiment with gas, in which about 25 Soviet prisoners were gassed to death in the basement of Block 11.[27] The experiments lasted longer than 20 hours

According to Rudolf Höss, the commandant of Auschwitz, Bunker 1 held 800 victims, and Bunker 2 held 1,200 victims. Once the gas chambers were full, their doors were closed tightly, and solid pellets of Zyklon B were dropped into the chambers through pipes in the side walls, thus releasing the cyanide gas immediately. Those victims inside the gas chambers usually died within 20 minutes. The speed of the deaths depended on how close the victim was standing to a poison gas vent, according to Höss, who estimated that about one third of the victims died practically immediately.

Johann Kremer, an SS doctor who oversaw the gassings, testified: the "shouting and screaming of the victims could be heard through the opening and it was clear that they fought for their lives".[31] If the gas chamber had been crowded, which they typically were, the corpses were found half-squatting, their skin discolored pink with red and green spots, with some found foaming at their mouths, or bleeding from their ears.

The product is infamous for its use by Nazi Germany to murder an estimated 1.2 million people, including approximately 960,000 Jews, in gas chambers installed in several extermination camps during the Holocaust. One of the co-inventors of Zyklon B, chemist and businessman Bruno Tesch, was executed by the British in 1946 for his role in this operation.