Semtex is a general-purpose plastic explosive. First made by the Semtin Glassworks (then called VCHZ Synthesia , now called Explosia ) in the former Czechoslovakia (now the Czech Republic). Used in commercial blasting and demolition and in certain military applications. Its notoriety is due to its popularity with terrorists because it was, until recently, extremely difficult to detect, easily obtained, and as little as 250 g could down an airliner (312 g in the case of Pan Am flight 103).

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

The explosive is named after Semtin, 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 C4, 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 tonnes. However, the main consumer was Libya; about 700 tonnes of Semtex were exported to Libya between 1975 and 1981 by Omnipol .

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

Also in response to international pressure, Semtex has ethylene glycol dinitrate added to produce a distinctive odour 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.