THE early work of Bäckström1 demonstrated the chain character of both the photochemical and the 'thermal', that is, dark, oxidation of benzaldehyde to peroxide in the liquid phase. We have studied the kinetics of the thermal reaction in benzene solution at 31° C., and have found it to be catalysed by di-benzoyl peroxide. The increase in rate Δρ above that of the uncatalysed reaction ρ0 is proportional to the square root of the concentration of dibenzoyl peroxide, as shown in the accompanying graph.