Environmental friendly water participating frequently as catalyst or reactant has been attracting much interest, and many acid-catalyzed organic syntheses have been investigated in hot water ranging in maximal ion product (K-n). However, not only are long reaction times required to obtain satisfactory yields, but side reactions are easy to occur, leading to poor selectivities. Our device of a microvolume reactor system using a quick-heating (QH) and quick-quenching (QQ) process that can heat up very quickly an ambient substrate solution to the supercritical water (scH(2)O) state and then can quench rapidly to sufficiently low temperatures after the reaction achieves nearly 100% selectivity and a satisfactory yield over 80% for the epsilon-caprolactam production at reaction times shorter than I s even in the absence of any acids. This reaction system also elicits a marked increase of the rate of reaction by about I 100 times in 7.69 in H2SO4. The acidic ability of H2O peculiar to supercritical conditions is considered to promote the desired pathmay only, and so the scH(2)O QH-QQ process prevents the hydrolysis and pyrolysis of cyclohexanone-oxime as well as the pyrolysis of epsilon-caprolactam. Our new approach further enables nearly 100% yield and 100% selectivity in the presence of dilute acid under SWQH conditions. (C) 2003 Elsevier Science Ltd. All rights reserved.