Li-6 and C-13 NMR on ethyl alpha-lithioisobutyrate (EiBLi) and quantum-chemical (DFT) calculations an methyl alpha-lithioisobutyrate (MiBLi) were used to elucidate the structure of the active center in the anionic polymerization of methacrylates in the presence of triethylaluminum (AlEt3) in toluene. This study reveals ester enolate/aluminum alkyl complexes with different degrees of association, (MiBLi AlEt3)(n) (n = 1, 2, 4), and different stoichiometries, MiBLi . xAlEt(3) (x = 1, 2). In the presence of methyl pivalate (MPiv), which is taken as a model compound for the monomer and polymer, complexes such as (MiBLi . MPiv . AlEt3)(n) (n = 1, 2) are formed. These complexes can dissociate into MiBLi . 2AlEt(3) and MPiv . AlEt3 when AlEt3 is used in excess.