The mechanism of thermal decomposition of 4-hydroxy-2-butanone in m-xylene solution was studied experimentally and theoretically at the M05-2X/6-31G(d, p) level of theory. It follows first-order kinetics and appears to be homogeneous and unimolecular. The proposed mechanism is via a six-membered cyclic transition state to give a mixture of formaldehyde and acetone. Rate constant values were experimentally determined at three temperatures: 483.15, 493.15, and 503.15 K. Calculated rate constants are of the same order of magnitude than the experimental ones. Calculated Gibbs energies of activation agree very well with the experimental values. Computationally, the progress of the reactions was followed by means of the Wiberg bond index. The results indicate that the transition state has an intermediate character between reactants and products, and the calculated synchronicity shows that the reaction is slightly asynchronous. The bond-breaking processes are more advanced than the bond-forming ones, indicating a bond deficiency in the transition state. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 407–413, 2012