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Semtex is a general-purpose plastic explosive. First made by the Semtín Glassworks (then called VCHZ Synthesia, now called Explosia) in Semtín (a suburb of Pardubice) in the former Czechoslovakia (now the Czech Republic), it is used in commercial blasting, demolition and in certain military applications. Semtex became notoriously popular with terrorists because it was, until recently, extremely difficult to detect and easily obtained. It is also very effective for air terrorism: as little as 250g could destroy a commercial passenger airplane [1], as in the case of Pan Am Flight 103 [2].

There are two common varieties, A for blasting and H (or SE) for hardening:

The explosive is named after Semtín, a suburb of Pardubice in eastern Bohemia where the compound was first manufactured. It was invented in 1966 by Stanislav Brebera, a chemist at VCHZ Synthesia. It was like other plastic explosives, especially C-4, in that it was easily malleable; but it was usable over a greater temperature range than other types.

The new explosive was widely exported, notably to the government of North Vietnam, which received over 12 tons. However, the main consumer was Libya; about 700 tonnes of Semtex were exported to Libya between 1975 and 1981 by Omnipol. It has also been used by Islamic militants in the Middle East and by republican paramilitaries such as the Provisional Irish Republican Army (IRA) and Irish National Liberation Army in Northern Ireland.

Exports fell after the name became closely associated with terrorist blasts. As of 2001, approximately only 10 tonnes of Semtex were produced annually, almost all for domestic use. Export of Semtex was progressively tightened and since 2002 all of Explosia's sales were controlled by a government ministry.

Also in response to international pressure, Semtex has ethylene glycol dinitrate added as a detection taggant to produce a distinctive vapor signature to aid detection. Efforts have also been made to reduce the shelf life of Semtex from its current 20 years to three or even less but have proved difficult, and all new supplies contain an identifying metallic code.

On May 25, 1997 Bohumil Sole, a scientist often said to have been involved with inventing Semtex, strapped the explosive to his body and committed suicide in the Priessnitz spa of Jeseník. Sole, 63, was treated there for depression. Twenty other people were hurt in the explosion, while six were seriously injured. It should be noted, however, that the manufacturers Explosia state that he was not a member of the team which developed Semtex.

Semtex is now manufactured in Brno, Czech Republic (about 10 tons per year).

• Website of the manufacturer with brief history of Semtex
• Manufacturer's rebuttal of various alleged misapprehensions, incl involvement of Bohumil Sole
• 2002 article in Christian Science Monitor on misuse of Semtex