Poly(o-aminobenzylphosphonic acid) and its sodium salts, made by progressive neutralization with NaOH, have been prepared and characterized. Both the hemisodium and monosodium salts are intrinsically self-doped and water-soluble in the conducting form. The key is to control the ionization of the excess acid functions on the polymer chains. XPS results suggest that the polymer is obtained directly in the 43% polyzwitterionic (i.e., self-doped) emeraldine form. The self-doped polymer does not exhibit any conductivity dependence on pH below 6, in marked contrast to the HCl-doped polyanilines. Our structurally well-defined polymer system displays only one set of redox activity over the potential range of -0.2 to 1.2 eV, alluding to the higher stability of the self-doped structure. This is unique and different from other polyaniline systems which show two sets of redox peaks. FTIR spectroscopy provides evidence for strong intra-chain hydrogen bonding while NMR studies conclude that protonated and unprotonated segments of the solvated monosodium polymer are distinct. The lower conductivity (similar to 10(-3) S cm(-1)) obtained is attributed to the decrease in conjugation caused by the large steric effect of the bulky PO3H2 and a significant hydrogen bond interaction between PO2(OH)(-) and NH.+ leading to significant charge-pinning. UV-vis data can be explained by a increased participation of the nitrogen wave function in the HOMO, and thereby the widening of the pi(B) ---> pi(S) band gap.