The synthesis and solution-state NMR investigation of the structure of a novel type of water-soluble polyaniline (PANI) derivative in leucoemeraldine and emeraldine oxidation states is presented. Copolymers of 3-aminobenzenesulfonic acid and 2-methoxyaniline (PMAOMA) with various monomer molar ratios were synthesized by chemical polymerization in a hydrochloric acid medium. Due to a large difference in the reactivities of 2-methoxyaniline (OMA) and 3-aminobenzenesulfonic acid (MA), the content of OMA in the copolymers was much higher (over 0.65 of the mole fraction) than in the reaction mixture. Structural characterization of the reaction products was performed using one- (H-1, C-13) and two-dimensional (GHSQC and GCOSY) NMR spectroscopy. The NMR data indicated an almost block structure of OMA sequences of variable length separated by shorter sequences of MA units. It was also shown that the incorporation of MA units in the copolymer chains favors the formation of a less oxidized material.