The mixed complexation of methyl alpha-lithioisobutyrate(MIBLi)(a monomeric model structurally corresponding to the living end of PMMA-Li+) with lithium tert-butoxide (LiOtBu) in THF has been studied for the first time by using C-13 NMR spectroscopy. It is shown that LiOtBu, like LiCl, is very effective in coordinating with MIBLi tetramer in THF, incrementally replacing MIBLi with the formation of other tetrameric complexes, MIBxLi4(OtBu)4-x (x = 1-3). Complextion of MIBLi by LiOtBu induces a shielding C-13 shift of the MIBLi C(alpha) resonance, which might suggest that lithium cations associated with the electron-rich ligands, i.e., LiOtBu, can also form weaker bonds with the carbonyl oxygen of MIBLi (i.e., mu-type complexes), thereby decreasing the localization of the negative charge on that latter atom. In comparison with LiCl, one of the most significant features is that only one single type of species results from mixed complexation of MIBLi and LiCl at LiCl/MIBLi = 1, 2, and greater-than-or-equal-to 3, respectively, at low temperature, whereas several LiOtBu-complexed species may coexist, whatever the LiOtBu/MIBLi ratio; furthermore, their exchange is found to be very slow at a temperature as high as 0-degrees-C. Tentatively, this striking feature may explain nicely the large difference in molecular weight distribution of poly(meth)acrylates anionically prepared in the presence of LiOtBu and LiCl, respectively.