A mechanism involving a six-membered cyclic transition state where the hydrogen of the hydroxyl group interacts with the oxygen of the carbonyl group has been proposed previously to describe the thermolysis of many beta-hydroxyl compounds. In this paper, the proposed mechanism is studied for a series of beta-hydroxyl aldehydes. Rate constants and activation energies are reported as well as a study of the influence of tunneling on the reaction rates. The electron density at the ring critical points, population analyses by the theory of atoms in molecules (AIM) and the natural bond orbital (NBO) method, as well as atomic energy analyses are used to gain insight into this interesting mechanism and into the effects of substituents.