The potentiometric titration, intrinsic viscosity, and C-13 NMR have been measured under various conditions to investigate the dissociation behavior of poly(2-methyleneglutaric acid) (PMGA), which has a charge density twice that of poly(acrylic acid) (PAA). The titration curves show that PMGA exhibits an apparent one-step dissociation similar to PAA, remarkably different from those of poly(maleic acid), poly(fumaric acid), and poly(itaconic acid), all having the same average charge density. The electrostatic interaction of PMGA does not level off at degrees of dissociation (alpha > 0.5), and the negative logarithm of the intrinsic dissociation constant (pK(0)) of PMGA is 4.20 +/- 0.05, similar to that of PAA. The intrinsic viscosity of a PMGA solution containing 0.05 N NaCl increases linearly with a. It has been shown from C-13 NMR measurements that the carboxyl groups on the side chain of PMGA dissociate preferentially in the region of alpha < 0.4. The electrostatic work required to remove H+ from the rod surface calculated from a smeared charge model is in good agreement with the experimental data of PMGA, only for alpha less than or equal to 0.5. The dissociation of carboxyl groups of PMGA in alpha > 0.5 is strongly depressed because of the decrease of the dielectric constant of water.