The effects of oxygen concentration on the stability of methacrylic acid (MAA) have been examined using HQ, MeHQ and PTZ as inhibitors. The O2 has been found to have a dual effect so that an optimum concentration confers very high stability but increased [O2]s give significantly reduced induction periods, especially at higher temperatures. This behavior has been explained in terms of radical scavanging competing with peroxide formation and cleavage. The main parameters controlling the system have been examined. The rate of stabilizer consumption was found to be controlled mainly by temperature and [O2]. The rate of energy release on polymerization is dependent on temperature, [stabilizer] and O2 supply to the system and is rapid (violent in extreme cases) only when all three factors are favorable for efficient peroxide formation and cleavage. Preliminary experiments with n-butyl methacrylate indicate that the same principles and mechanism operate for the simple methacrylate esters.