The aim of this paper is to explore the reason for the catalyst-free phenomenon of synthesizing 4-hydroxy-2-butanone from formaldehyde and acetone under supercritical conditions and in high-temperature liquid-phase. Experiments were conducted in a tubular flow-type reactor at temperatures between 230 and 270 degrees C for residence times between 0.5 and 8 min. Under these conditions, a small amount of formic acid appeared via noncatalytic self-disproportionation of formaldehyde. The generated formic acid proved to play a role as catalyst in main reactions. A mechanism-based detailed chemical kinetics model for main reactions was suggested to assess the effect of temperature. This model applied well both under supercritical conditions and in high-temperature liquid-phase. The activation energies for forming and dehydration of 4-hydroxy-2-butanone were 97.5 +/- 3.5 kj/mol and 120.6 +/- 5.8 kJ/mol respectively. (C) 2015 Published by Elsevier Ltd.