The effect of air on atom transfer radical polymerization (ATRP) of phenethyl methacrylate (PEMA) was investigated. The results indicated that air alone induced polymerization in the absence of an added initiator with the ATRP catalyst complexes NiBr2(Pn-Bu-3)(2) and Cu(n)X-n/ligand (n = 1, 2; X = Cl, Br; ligand = dipyridyl (dpy) or pentamethyldiethylenetriamine (PMDETA)) at high temperatures. Molecular weights of the polymers were not predictable, but the polydispersities were low. Polymers with very low polydispersities (1.08) and high molecular weights (704 000) were obtained with CuCl2/PMDETA catalyst complex. The results indicate that a reverse ATRP mechanism is probably involved in the air-induced polymerization. The extension of the air-induced polymerization to n-butyl methacrylate shows that the air-induced polymerization should be generic for methacrylates.