Several researches have dealt with the thermal initiation of Me methacrylate (MMA) in the past. Some of them already discussed the presence of peroxide contg. species that are formed from dissolved oxygen and the monomer itself as main reason for this initiation. However, a more detailed investigation as well as a kinetic description of this phenomenon is still due in literature. In this paper, the formation and decompn. of Me methacrylate peroxides are described. MMA that has been in contact with air forms macromol. peroxides at temps. below 100 DegC from phys. dissolved oxygen. These peroxides have mol. wts. of approx. 3 000-5 000 g . mol-1, depending on the temp. during formation. Above this temp., these peroxides decomp. quickly and initiate the radical polymn. Depending on the reaction conditions, monomer conversions from 15 to 30% are obtained. In combination with addnl. initiators, the MMA peroxides provoke an acceleration of the reaction rate and can also lead to bimodal mol. wt. distributions. An anal. method based on UV-spectrophotometry was developed for the quantification of the peroxide content in the monomer. The kinetic rate consts. for the formation were detd. in batch expts. with purified, air-satd. monomer to be kf,0 = 6.28 . 107 l2 . mol-2 . s and EA = 7.75 . 104 J . mol-1. The decompn. rate consts. were detd. from batch dead-end polymns. and found to be kd,0 = 4.73 . 107 l . mol-1 . s-1 and EA = 8.56 . 104 J . mol-1.