Polyelectrolyte behavior and cooperativity of electrostatic coupling of two weakly charged polyions, poly(sodium methacrylate)-co-(acrylamide) containing 10 mol % ionic groups (A10) and poly(N-diallyldimethyl-ammonium chloride)-co-(acrylamide) containing 8 mol % ionic groups (C8), was examined using H-1, Na-23, and Cl-35 NMR and H-1 PGSE NMR, ab initio SCF HF/6-31G(d) calculations, and simulations using a simplified theoretical model of insular equilibria. With the use of Na-23 and Cl-35 NMR relaxations, it was first shown that neither A10 nor C8 exhibits polyelectrolyte effects in the sense of counterion condensation in dilute aqueous solution. Accordingly, electrostatic binding of low-molecular-weight models of the charged groups, namely sodium pivalate (for A10) and tetramethylammonium chloride (for C8), to the complementary polyion is weakly populated and exhibits no cooperativity (as shown by Na-23 and 15 Cl NMR relaxations and H-1 PGSE NMR). However, interaction of A10 with C8 in dilute solutions is distinctly cooperative and leads to relatively high coupling degrees. A simplified theoretical model based on a restriction imposed on the motion of the coupling groups by neighboring already coupled pairs, which we present here, explains this behavior and predicts experimental data in a semiquantitative way. The model can be improved by including a detailed description of motional constraints.