The solubility properties of amorphous poly[2,2-dicyano-1-[242-(methacryloyloxy)ethyldimethylammonio)ethoxy]-ethenolate] (zwitterionic lateral group of mu = 25.9 D) investigated in 43 solvents of widely differing polarity may be interpreted in terms of solvent molar refractivity (dispersion interactions), dipole moment (dipolar interactions), basicity (donor-acceptor interactions with the ammonium site) and hydrogen-bond-donating power (hydrogen bonding to the anionic site). In dimethylformamide, representative of strongly dipolar aprotic media, the phase diagram is characterized by a lower critical solution temperature corresponding to a lower theta temperature of about 65.8 +/- 1-degrees-C. Coil expansion at 25-degrees-C, as derived from intrinsic viscosity measurements, is an increasing function of the concentration of added lithium salts (less-than-or-equal-to 1.5 mol l- 1), and the order of anion efficiency is Cl- > Br - > I- > ClO4-. In trifluoroethanol, representative of weakly associated and strongly protic solvents, the addition of tetramethylammonium salts (less-than-or-equal-to 1 mol l-1) results in coil expansion only in the case of the more polarizable ClO4- and I- anions, while Cl- and Br- promote chain collapse. The hydrophobic polyzwitterion becomes water soluble at 25-degrees-C only in the presence of high salt concentrations, such as NaSCN > 2.5 mol l-1, ZnCl2 > 5 mol l-1, C2H5NH3+NO3- > 6 mol l-1, and the phase diagram is characterized by an upper critical solution temperature strongly dependent on salt concentration (dT/d[NaSCN] = -70-degrees-C 1 mol-1). In strongly acidic media, such as concentrated H2SO4, HClO4 aqueous solution (40-60 wt%) or methanesulfonic acid, partial protonation of the anionic sites transforms the polyzwitterion into a water soluble cationic polyelectrolyte.