Mn(2+), Cu(2+), and nitroxyl amines have been shown to bond to plant homopolygalacturonan matrices in a spatially sequential fashion. As a consequence of this special form of cooperativity the lattice constant (kappa), determined from Van Vleck's second moment relationship, approaches 1 only when the average number of dipolar interactions per spin approaches 1 (e.g., an array of dimers). Assuming that one paramagnetic ion or nitroxyl amide pair is bonded per polymer block within the matrix when kappa = 1, the anionic ligand's average degree of polymerization ([unk]) can be estimated from the concentration of bonded paramagnetic dimers (e.g., [1/chi](kappa approximately 1) = [unk]; chi is the mole fraction of bonded paramagnetic dimers). We have utilized this technique to estimate the average molecular size of homopolygalacturonan blocks in intact higher plant cortical cell walls ([unk] approximately 83), Nitella cell walls ([unk] approximately 27) and a commercially available galacturonic acid polymer ([unk] approximately 35). The [unk] determined from both the intact cortical cell wall lattice and the polygalacturonan were similar to literature values; these findings argue that the electron paramagnetic resonance, (EPR) dipolar spin-spin interaction technique reported herein is a valid approach for estimating molecular size in plant cell walls.