The p(MMA-b-t-BuA) copolymer was prepared using the initiation system methyl 2-lithioisobutyrate/lithium/tert-butoxide (MIB-Li/t-BuOLi). A low-molecular-weight fraction was isolated and analyzed by MALDI-TOF mass spectrometry with the aim to find out structures of products of self-termination, which proceeds mainly after the addition of t-BuA into the living PMMA. The main self-termination products are formed after addition of one or two t-BuA units onto living PMMA-Li by backbiting reaction of t-BuA-Li chain-ends and carbonyl group of the antepenultimate MMA units of the same chain. The oxoesters generated in this way contain hydroxygroup, and its hydrogen can protonize any of the living chains, regardless of the type of its terminal unit. Signals of the chains, terminated by this protonization, have also been found in the spectra. Thus, the first direct evidences for the structures of the self-termination products in the anionic methacrylate/acrylate block copolymerization were obtained in this way.