The acetaldehyde pyrolysis has been studied over the 480–540 °C temperature range, and at pressures from 1 to 560 mm Hg. Measurements were made of the rates of production of the major product methane and of the minor products hydrogen, ethane, acetone, and propionaldehyde. The kinetic effects of adding carbon dioxide, of changing the surface:volume ratio, and of coating the vessel have been determined. The elementary processes are found to occur largely in the gas phase. The dissociation of CH3CHO into CH3 + CHO is deduced from the rates of hydrogen and ethane formation; there is significant falling off of the rate constants at lower pressures. The kinetic parameters for the reaction CH3 + CH3CHO → CH4 + CH3CO are obtained, and it is suggested that quantum-mechanical tunnelling plays a role. Acetone production has an activation energy of 12.4 kcal/mole and it is concluded that its formation involves the participation of the isopropoxy radical, the activation energies being as indicated:[Formula: see text]