High-pressure and high-temperature water (HPHT-H2O) microreaction system was proven to be an efficient method to carry out the non-catalytic Claisen rearrangement. Allyl phenyl ether undergoes Claisen rearrangement to obtain o-allylphenol with a very high yield and selectivity of 98% within a short reaction time of 13.4s at 265 degrees C and 5 MPa pressure. Compare to the solvent-free microreaction. HPHT-H2O plays an important role to accelerate the reaction as a catalyst by transferring a proton along locally formed hydrogen bond with the substrate. Finally, the obtained product can be separated completely from the aqueous suspension within the time range of 40 min to 24 h after the reaction. Even though the reaction has occurred in aqueous conditions, little side reactions, such as hydrolysis, hydration, or pyrolysis. was found. (C) 2010 Elsevier By. All rights reserved.