A one-step synthesis of monoterpene alcohols such as lavandulol (Scheme3, 5), linalool(8), and geraniol(7)/nerol(10) and a-terpineol(6) from hemiterpene alcohols proceeds efficiently in supercritical water (scH(2)O) without any catalyst. Our device of a microreaction (mu-reaction) system that can heat up very quickly an ambient substrate solution to the scH(2)O state and then can quench rapidly to sufficiently low temperatures after the reaction achieves the selectivity to (5) and the yield reached 82 % and 59 %, respectively; even at a short reaction time of 7.7 s at 30 MPa and 375degreesC in the near-critical region. The yield was superior to the conventional method promoted by organometallic catalysts in which the reaction time of 7.25 h is needed to obtain 54 % yield. When the reaction was carried out at a lower temperature of 300degreesC in hot water, we obtained only 15 % yield even at a reaction time of 38 s. The acidic ability of H2O peculiar to supercritical conditions is thus considered to promote the desired pathway alone, and so the scH(2)O p-reaction system could conveniently prevent side reactions such as dehydration and hydrolysis. It is further noteworthy that the presence of methanol in a high-temperature water medium completely suppresses the (5) production. This seems to be due to its own acidic ability peculiar to scH(2)O state. (C) 2004 Elsevier Ltd. All rights reserved.