AbstractSolutions of polymethacrylic acid have been partially neutralized to different extents, and the pH values and viscosities of the resulting solutions determined. These measurements were carried out at several different potassium chloride concentrations, and on several polymer samples. The calculated dissociation constants of the polymer acid were substituted in the equation developed by J. Th. G. Overbeek, which expresses the change in dissociation constant of a polybasic acid as a function of the radius of the molecular coil and of the charge on the molecule. The derivation of this equation treats the polymer molecule as a sphere containing a continuous charge. The radii found are of the right order and change in the expected way with changing ionic strength and ionisation. Substitution of these radii in the Debije viscosity equation gives satisfactory agreement between observed and experimental viscosities at low degrees of ionisation. At higher degrees of dissociation it would seem that the treatment of the polymer coil as a sphere is no longer satisfactory, although a better approximation is obtained if the charge is considered to be entirely on the surface of the sphere.