AbstractA theoretical analysis is given on the effect of low molecular salts on the titration behavior of partially neutralized linear polyacids or polybases. On the basis of the additivity law experimentally and theoretically established on the activity of counterions in polyelectrolyte solutions containing salts, the chemical potential of alkali added for neutralization of polyacids is derived as a function of the salt concentration. The relation between the pH and the salt, concentration at constant degree of neutralization is expressed by pH(cs) − pH(0) = ({[∂ (γpCa)/∂Ca]/γos} − 1) ln [1 + (γosCs/γpCA)] where Ca is the concentration of alkali, C8, the concentration of salts, γp is the activity coefficient of counterions in the absence of salts and γos is that in the absence of polyelectrolytes. This relation is found to be in good agreement with experimental data obtained in synthetic linear polyelectrolytes as well as in rodlike charged proteins. Therefore, it is concluded that the titration behavior of linear polyions can be understood from the special feature of the integrated coulomb interaction between linear polyions and small ions which was essential for the additivity law. In addition, the screening effect of salts on the electric potential around the polyion is discussed.