Water sorption by amorphous (meth)acrylic poly(zwitterions)-bearing quaternary ammonioalkoxydicyanoethenolate side groups drop N+-(CH2)(p)-O-CO-C-(CN)(2), dipole moment mu(D) = 25.9 and 30.8 for p = 2 and 3, respectively was studied at 23 degrees C over a broad range of water activity a (0.14-0.98). Water diffusion is nearly Fickian (D-s = 5.9 10(-)7 cm(2).s(-)1 for a = 0.63) and the sorption isotherms may be quantitatively analyzed according to the Guggenheim-Anderson-De Beer equation for a multilayer process characterized by a number of site-bound water molecules per monomeric unit, n(m) similar to 0.7. The Flory chi interaction parameter is a strongly increasing function of the water content in the glassy hydrated systems and it always remains higher than 0.75. Clustering of water molecules (Zimm-Lundberg theory) is never observed. Differential scanning calorimetry allows to quantify nonfreezable bound water (type I) of strong plasticization efficiency, n(I) = 2.8 mol. of water per monomeric unit, and it points out the quasisimultaneous emergence of low amounts of freezable bound water (type II) crystallizing at -40 degrees C and melting at -1 degrees C and of bulk-free water (type III, n(II)/n(III) similar to 0.1). All these typical features distinguish these rather hydrophobic poly(zwitterions) from their hydrophilic homologues of the quaternary ammoniopropanesulfonate type (drop N+(CH2)(3)-SO3,-mu(D) = 24, n(m) = 1.5, n(I) = 6.7).