The steady-state γ-radiolysis of aqueous solutions containing 1x10 - 3 mol dm - 3 methyl ethyl ketone (MEK) has been studied at a dose rate of 0.12 Gy s - 1 , 25 o C and an initial pH of 10. Experiments were conducted in air-, Ar- or N 2 O-purged aqueous solutions, or in Ar-purged solutions with added tert-butanol. MEK, its radiolytic products, and the change in pH resulting from MEK decomposition were analysed as a function of time (or total absorbed dose). The main initial step for the radiolytic decomposition of MEK is the H abstraction from MEK by OH, produced by γ-radiolysis of water, to form MEK radical. In the absence of O, the main decay path of the MEK radical appears to be dimerization to form 3,4-dimethyl-2,5-hexanedione. In the presence of oxygen, the MEK radical reacts primarily with O to form the MEK peroxyl radical. This radical ultimately results in a series of progressively smaller oxidation products. The formation of organic acids, and eventually CO 2 , reduces the pH of the solution. This paper presents the experimental data and proposes the MEK decay kinetics and mechanism.