This paper presents the results of case studies on accidental gas explosions in the history of advanced technologies using highly reactive gases in Japan. In November 1984, a reservoir cylinder containing mono-germane imported from the United States exploded. This was the first serious explosion due to mono-germane decomposition in Japan. Five years later another explosion due to mono-germane decomposition occurred. These two accidental explosions forced the Japanese government to investigate the decomposition characteristics of semiconductor manufacturing gases. Consequently, it was found that mono-germane decomposed by adding a small amount of energy. Although pure mono-silane does not explode, its mixture with oxidizer explodes in a wide range of mixture compositions. The most serious such accidental explosion occurred at a laboratory of a university in October 1991. The gas which exploded in this case was a mixture of mono-silane and nitrous oxide. One of the major causes was a lack of knowledge on sealing in the check-valve. Besides the explosions, the causes of which were unknown before, explosions of well-known causes have occurred. Based on knowledge accumulated through these accidental explosions, the technologies against explosions have been established for semiconductor tip industries. (C) 2003 Elsevier Ltd. All rights reserved.